Without having read Mises or Hayek myself and only having a general grasp of their theory and of econ, was there anything they got right about the inefficiencies/implausibility of socialist economics? To what extent did the shortcomings of socialist countries' economies have to do with what they predicted with the ECP?

Considering the successes of socialist countries I'm guessing the answer is "not really", but I'm confused why they were wrong if that's the case. What is it that the planners of China, USSR, etc did that basically allowed them to (seemingly) sidestep Mises' and Hayek's theory?

I'm aware that the calculation debate is pretty much solved in the 21st century with modern computing and data processing technology but I'm talking about the 20th century socialist countries and their shortcomings. I think Hayek acknowledged you could determine the supply/demand equilibrium mathematically instead of having to rely on markets and "invisible hand", but he still found planning to be implausible. Was he wrong, and if so why? (In the context of 20th century socialist economics)

Also please recommend me any books/resources on socialist economics if you have any, thanks.