Unsurprisingly, this is a very rich question, but I will cover the case of the Soviet Union generally. First we probably want to start with definitions: "unbiased" research does not exist. There are overtly slanted readings of Soviet history - ones that are explicitly anti-communist, ones that are unapologetically sympathetic to the Soviet cause. The former might elide, say, the very real scientific accomplishments of the USSR by emphasizing the absurdity of Lysenkoism; the latter might understate the intentionality and scale of the Holodomor. But any historical argument prioritizes some evidence, and advances an interpretation open to debate.

When you reference the “poverty or unsustainable nature of large communist governments such as the USSR,” it’s worth pulling out what time period you’re talking about: immediately before the fall of the Soviet Union (so, to what extent did foreign sanctions precipitate the fall of the Soviet Union?), or the presence of extreme poverty immediately following the revolution until the death of Stalin.

I would not place foreign sanctions at the center of Soviet famines in the 1930s, for example, nor for the collapsing industrial production of the 1920s (though it’s worth considering the relationship between Russian exports of raw material and the economies of trade partners after WWI, as well as during the Depression - in the early 1930s Soviet exports are indeed restricted in Britain and the US, punctuated by British sanctions following the Metro-Vickers affair).

The late Soviet Union is quite a different beast than the high Stalin era; the extreme poverty of the 1930s has been supplanted by a decent standard of living for the vast majority of Soviets, coupled with far more concerted sanctions and embargoes. From 1979 to 1991 the United States exerted large pressure on the Soviet Union through sanctions, particularly pronounced after 1983, but again I would not point to these as a significant and certainty not a proximate cause of the collapse of the USSR.

Peter Schweizer's Victory: The Reagan Administration's Secret Strategy is well regarded in some circles, and advances the claim that sanctions were in fact successful. Schweizer, however, greatly misses what was happening internally in the stagnating Soviet Union - social pressures, internal political mismanagement, and so on. I would, in response to the economic question, highlight the Soviet reliance on oil which collapsed during the 1980s. The political scientist Robert Pape writes that of 40 sanctions events studied "there are only five instances...in which economic sanctions were clearly successful” (including the 1933 British sanctions on the Soviet Union). A conventional reading of the collapse of the Soviet Union highlights economic stagnation, the Soviet invasion/intervention of Afghanistan, corruption, and general social dissatisfaction accelerated by glasnost and perestroika.