I'll read the words of Uncle Ho!
For the Trots and MLMs, as I said in another reply, this has to do with regional difference.
Being next to the USSR and other Eastern Bloc countries as an imperialist puppet regime must take its toll on the people. And the founding principles of the country had anti-communism in them (communism must be crushed wherever it is seen.)
As for Cuba and Vietnam, they were in similar conditions to us. They succeeded; we failed.
Iirc a famous streamer in my country was talking about the Maoist movements in the Global South (Japan, India, Türkiye, etc.) and Trotsky denouncing the Trotskyists. And criticized Stalin from the points I wrote in the post. He put that stream behind a paywall, so I can't rewatch it for now.