Khrushchev, Beria, Yezhov, etc. It really feels like they didn’t do much quality control, if at all. I know Stalin was, contrary to popular belief, quite trusting. Especially in regards to Bukharin and the likes. But it’s not like he had the power to really kill anyone he wanted, again contrary to popular belief, even if he wanted to (at least any politburo members), so his personal views of his colleagues don’t matter.
Well one reason a lot of the most ideologically dedicated communists were the first to die fighting the Nazis and the USSR was forced to lower their standards of who was let in order to win the war. China today by contrast has a massive population it can easily afford to cherry pick from. Another is the very chaotic manner in which the USSR emerged, trying to remove every impure individual and faction would of meant a longer civil war which couldn’t be afforded with Western powers on the doorstep, things needed to start getting rebuilt asap and to do that you eventually need to make peace and confederate with some less desirables, which there were plenty of given that the USSR had just emerged out of the reactionary remains of the Russian Empire.