Is the U.S. really the main reason why Cuba’s economy is struggling?

Since Cuba became socialist and received support from the USSR, the U.S. has put tariffs on Cuba for the majority of goods, except for some medical stuff and other essential goods.

Other countries, such as China, Russia and European countries can trade with Cuba. The problem is that if a company residing in one of those countries trades with the U.S. and with Cuba at the same time, they risk that tariffs targets them as well, incentivizing to leave Cuba out of the trade. Most of the trades are made in dollars too, so financial transaction involving US banks are restricted.

However, we also know that socialism does not work, and that countries targeted by US embargo such as China in the 70s or Vietnam, became successful only when introducing capitalism (free markets and private property of means of production).

So the question is, even without US embargo, would Cuba still fail?