The ARVN gets a bad rap, primarily from veterans who largely did not see them in action, and from historians who misunderstand Vietnamese internal culture and their role in the post-Vietnamization era of the war. Now, don't get me wrong, much of it is deserved, but to understand the role of the ARVN in the war and how they performed requires a more nuanced look than many histories have time to provide.
The core of the PAVN, and even the NLF, was formed from Viet Minh leadership and veterans. Now, that's not to say that there weren't Viet Minh veterans who had joined the Southern side, but there was an effective core of leadership from that era who were leading troops against the South.
Initially, the ARVN were formed in the same image as the French army in Vietnam. Mainly organized around light infantry, with little armor they had organized into cavalry scout units. Now this was largely appropriate for fighting a counterinsurgency, as the French themselves had largely done, but with the French out and the Americans in, there was a new strategic thinking—the new threat with an independent North Vietnam was presumed to be a conventional invasion, aided by the insurgency in the South.
Thus, the ARVN were reorganized into the structure we are most familiar with, which is the American-style layout of heavy, mechanized infantry divisions. The South Vietnamese would learn to use their tanks and APCs as infantry support, while also adopting US airmobile tactics, as well as the whole idea of overhwhelming artillery and air support. Once the war started heating up however, the Americans decided they would adopt the major combat operations, while relegating the South Vietnamese to guarding major population centers, after a few major and quite frankly embarrassing defeats.
Now, while some of these defeats can be attributed to poor morale and training, as the draftee army was, mainly due to the regionally based recruitment methods that many South Vietnamese divisions adopted. At this early point in the war, we can point fingers at their commanders. Now, the ARVN is notorious for its corrupt and ineffective commanders, and early war, this is definitely true. Patronage played a huge role in the appointment of officers, who were often promoted on the basis of loyalty to whomever was head of state at the time—the constant coups in South Vietnam led to a system that encouraged generals to either stay in their lanes and not rock the boat, or to suck up to the President/Prime Minister to ensure their position.
Many times, this corruption led officers to be reluctant to lead their troops in more aggressive actions, as the money they earned on the side siphoning supplies, etc. were much more lucrative operations than their actual jobs. It's also important to note that pay in the ARVN was abysmal for everyone, which in turn also encouraged the idea of side gigs in the army.
Now, the reputation of the ARVN as cowardly and corrupt had some true streaks, but overall it was remarkably average. While it had good units and bad, the units considered "elite:" the Marine Division, Airborne Division, and the Rangers, were considered equal to American units in morale, aggression, and effectiveness. In addition, some ARVN regular army units were also considered very good, such as the 1st Division, which was called by Lt. Gen. Richard G. Stilwell the division was “equal to any American unit.”
Its commander, Ngo Quang Truong, was called a “tough, seasoned, fighting leader” and “probably the best field commander in South Vietnam,” and General Creighton Abrams told his subordinates that he thought Truong was capable of commanding an American division.
Besides Truong, there was also a few other major South Vietnamese commanders that were considered excellent commanders, such as General Do Cao Tri, who earned the sobriquet, "Patton of Parrot's Beak," and General Le Van Hung, the Hero of An Loc.
Now, by the Tet Offensive in 1968, reform elements in the ARVN JGS had started to root out much of the corruption and combat ineffectiveness that had formally run rampant through the ranks, though desertion among common enlisted troops still remained a major issue. The ARVN performance during the fighting was a sign of this, like the soldiers fighting in the Hue Citadel.
That is not to say, that the ARVN still did not have major problems in conventional operations—they remained a top heavy force—favoring the American style of heavy artillery and air support to support mechanized forces. Lam Son 719, the invasion of Laos, was a notable sign of this, where South Vietnamese forces overextended themselves and failed to accomplish their objectives.
Now here we get to the primary weaknesses of the ARVN:
With the American withdrawal of combat forces in 1973, the South Vietnamese were left with an air force, which hugely competent, were under equipped to provide the same level and caliber of air support as the Americans once did. Importantly, there were no B-52s to provide hugely effective runs against massed conventional enemy forces as was possible during the Easter Offensive in 1972.
They relied on the US as its main source for its supply chain and logistics. Unfortunately for them, they had been trained to fight a "rich man's war," utilizing airmobile troops, massive artillery support, and overwhelming air support. South Vietnamese soldiers, originally functioning as light infantry, marching to battle, grew accustomed to the ease of mechanization with helicopters and trucks and armored personnel carriers.
Cuts to military aid by following the Paris Peace Accords, which dictated strict 1:1 supply terms meant that the ARVN were in bad supply straits: (I've pasted some info from a previous comment I made)
In the dire days of 1974 and 1975, the South Vietnamese were out of artillery shells (6 shells for 105mm pieces, 4 for 175mm pieces, per gun, per day) out of ammunition (rationing hand grenades and a 50% reduction to small arms ammunitions), out of spare parts for vehicles and equipment, meaning less air support and helicopter insertions, and even medical supplies, with anecdotal stories of the ARVN reusing bandages. This demoralized the ARVN, who from their perspective, saw no such cuts to the North Vietnamese. PAVN Chief of Staff, Van Tien Dung, commander of the 1975 Spring Offensive, observed that the firepower of the ARVN had been cut by 60% and their mobility by 50%.
American promises to intervene if the accords were seriously violated (which they definitely were by the North Vietnamese) lulled the South Vietnamese into believing aid or even American bombers were still coming, up until the very end.