Often lost in the discussion of Portuguese counterinsurgency is that it was not one but three conflicts centered around the major colonial holdings: modern-day Guinea-Bissau and Cape Verde, Mozambique, and Angola. While Angola was certainly a success militarily and Mozambique was going decently, the conflict in Guinea did not go particularly well and, arguably, was not much more effective than Vietnam. Below I'll discuss the Portuguese strategy and then go into some particulars about why some of the conflicts went better than others.

Portugal had been worried about a colonial war for some time. The Portuguese leadership was anxious about the spread of communism and the decolonization process by the other european powers resulting in a loss of their prized colonial holdings. This anxiety the creation of a formal counterinsurgency doctrine in 1960 called O Exército na Guerra Subversiva. Much like other counterinsurgency doctrines it emphasized population-centric action, a separation of insurgents from the general population via relocation and community self-defense. The doctrine is important to shaping Portuguese action during the subsequent conflicts because it incorporated lessons from the French and British experiences in colonial counterinsurgency and was completed the year before the colonial wars began in 1961.

In a somewhat ironic twist, Portugal's meagre resources relative to the US, UK, and France actually helped their strategy on the ground. The logic underpinning Portuguese action was to keep costs low enough to sustain the wars as long as necessary. A focus on affordability lead to some interesting efforts, such as the creation of cavalry units to patrol vast distances on the cheap and extensive recruitment of indigenous soldiers and auxiliaries to lower the costs of transporting troops from Portugal itself. The Portuguese also maintained a decent intelligence network that successfully infiltrated a number of rebel groups to spur internal strife and defections.

The Portuguese were also pretty successful in some instances of winning - or at least not losing - the hearts and minds of the local population. The Portuguese built more schools and medical facilities than they ever had during earlier colonial times. Often army manpower would be used on these projects, and Portuguese soldiers would be instructed to work as teachers, doctors, or construction workers during their off-time. Portugal also could not cause as much collateral damage as the united states did in vietnam because they did not have the airforce or the munitions to fight that way.

All that said, lets move into the individual cases, where we can look at specific factors instead of making generalizations.

Angola

Angola was the most successful example of Portuguese doctrine in action, but there were also favorable external conditions that helped the Portuguese do as well as they did. For starters, the opposition was disjointed. While it was the UPA/FNLA rebel group that had started the conflict, there was also the communist MPLA and the FNLA-breakaway group UNITA. These groups did not get along with each other and had a ton of internal divisions, which made them susceptible to infiltration and exploitation by the Portuguese intelligence services.

Additionally, the Angolan rebel groups were in a bad neighborhood to wage a counterinsurgency. Portugal successfully used its economic leverage to pressure neighboring Zaire and Zambia to not overtly support the rebel groups, although they were often used as shelter by the rebels. Additionally, South Africa was also waging a counterinsurgency and worked with the Portuguese by using its air power to help patrol southern and eastern sections of Angola.

These factors all came together in favor of the Portuguese and resulted in them more-or-less having the situation contained by the late 1960s.

Mozambique

Mozambique was less successful than Angola but is generally considered to have been salvageable. Mozambique also had a disunited opposition that struggled to win over the local population, and neighboring Rhodesia and South Africa actively resisted the rebels using their countries as rear bases.

One of the main problems with the Mozambique conflict was that some of the leadership was not on board with the low-cost hearts and minds doctrine. While the FRELIMO rebel group had been fairly contained in the north of the country using the established doctrine, in 1970 General Kaulza de Arriaga took command of Portuguese forces and went on a different path entirely. De Arraiga, a giant fan of U.S. general Westmoreland, masterminded operation "Gordian Knot," by far the largest operation by the Portuguese in the Colonial war, which used thousands of troops in an effort to rout FRELIMO and end the conflict. The result was much like Vietnam, tactical success and strategic disaster. FRELIMO didn't do very well but remained intact and many of its fighters were able to slip away and reestablish themselves in central Mozambique. The operation was expensive and displaced a ton of people. By 1974 it was a stalemate, with FRELIMO growing but not exactly beloved by the population, and the Portugese working to get a handle on the situation.

Guinea-Bissau and Cape Verde

This conflict was a lost cause for the Portuguese by 1974 and begs the question if their doctrine was actually good or if they enjoyed favorable circumstances in Angola and Mozambique. Unlike Angola and Mozambique, neighboring Guinea and Senegal couldn't care less about Portugal and actively supported the rebels. There was also only one major rebel group, the PAIGC, which was fairly united and pretty well-led, at least relative to the other groups Portugal was fighting.

They also couldn't move the population away from the rebels. In Angola and Mozambique there was enough space that when the rebels occupied an area, the Portuguese could relocate the population away from the rebels to prevent them from spreading their ideology and recruiting. Guinea was too small for this, so the population was more frequently in contact with rebels. Additionally, the terrain was very difficult for the Portuguese, and flooding and other factors undermined their ability to build infrastructure to win over the local population or effectively supply/reinforce their outposts.

Most agree that even if there wasn't a coup in Portugal in 1974, they would probably have lost this one. In 1973 the CIA drafted a memo to US secretary of state Kissinger on the state of Portugal's counterinsurgency, and claim Portuguese officials were privately admitting that the conflict may soon be lost.

Conclusion

I think the last thing that needs to be brought up is that the colonial war put enormous pressure on the Portuguese government and society. Portugal completely reshaped and reoriented its military to fight this war and withdrew most of the troops it had committed to NATO on the border with the Warsaw Pact. To fight these wars Portugal needed to implement extensive conscription and ended up losing five times as many men proportionally as the US did in Vietnam. Spending ballooned even while they were trying to keep costs low, and the coup that eventual ended the Estado Novo regime and the colonial wars was led by disaffected junior officers who had in some cases served tours of something like eight years at a time. So while some claim that Portugal did a better job in these wars, we need to consider what it cost them financially and socially to do so.

Bibliography:

Cann, John P. Counterinsurgency in Africa: The Portuguese Way of War 1961-1974. Solihull:Helion & Company. 2012. If you want a good overview of the conflicts with more details and you don't speak Portuguese, this is the book to get

Cann, John P. “The Artful Use of National Power: Portuguese Angola (1961-1974)”, Small Wars and Insurgencies 22:1 (2011), 196-225. https://doi.org/10.1080/09592318.2011.546616

Cann, John P. and Jose Manuel Correia. “An Unlikely Alliance: Portuguese and South African Airpower in Angola, 1968-1974.” Small Wars and Insurgencies 28:2 (2017): 309-336. http://www.tandfonline.com/doi/full/10.1080/09592318.2017.1288402

Engelhardt, Michael J. “Democracies, Dictatorships and Counterinsurgency: Does Regime Type Really Matter?” Conflict Quarterly 12:1 (1992), 52-63. https://journals.lib.unb.ca/index.php/JCS/article/view/15048/16117.

Henriksen, Thomas H. “Lessons from Portugal’s Counter-insurgency Operations in Africa.” The RUSI Journal, 123:2 (1978): 31-35. http://www.tandfonline.com/doi/abs/10.1080/03071847809422892.

Horan, Harold E. “Portugal in Africa.” CIA Reading Room. (1973). https://www.cia.gov/library/readingroom/docs/LOC-HAK-538-2-7-5.pdf

Paul, Christopher, Colin P. Clarke, Beth Grill, and Molly Dunigan. Paths to Victory: Detailed Insurgency Case Studies. RAND Corporation, 2013. http://www.jstor.org/stable/10.7249/j.ctt5hhsjk

Reis, Bruno C. and Pedro A. Oliviera. “Cutting Heads or Winning Hearts: Late Colonial Portuguese Counterinsurgency and the Wiriyanu Massacre of 1972.” Civil Wars 14:1 (2012):80-103. http://www.tandfonline.com/doi/abs/10.1080/13698249.2012.654690.

"Far more effective" is a poorly defined and mostly inaccurate term.

The Americans killed more enemy and held their place in the country longer.

The only way I believe you could consider the Portuguese involvement more effective in any way was it was less expensive, in lives and in gold. Other than that, both countries' campaigns were failures.