I am not that sure about Cuba. Their national currency is not very strong, and it may create issues for the government if Bitcoin is made a legal tender. That said, ordinary Cubans may find it easier to receive remittance payments, as sending them to Cuba in the form of fiat is not very easy (thanks to sanctions and embargoes). The same is the case with Iran as well. Initially the regime there supported Bitcoin mining, because they thought it will help them in circumventing the sanctions imposed by the United States administration.
I think just the opposite, their money is worthless so they should be taking bitcoin as legal tender. That way instead of having a fiat that is losing value constantly, they will replace it with bitcoin that gains value over time, and they would get richer instead of poorer. This will also allow more and more bitcoin in the nation, both in the pockets of the regular people but also it will allow government to gather more and more, even if it is just for tax payers that pay in bitcoin, they will have a stronger treasury as well. This is why when you are doing as bad as Cuba financially, it just makes sense to move to bitcoin legal tender, that way your nation will become richer and richer over course of years.
That certainly lies with them. if they will trust Bitcoin that could help them in the future that somewhat be possible but if they remain innocent and doubtful with this, I don't think this gives some room to Cuba or Venezuela. The corrupt government that they have might give them in trouble as I know they will take action for everyone who holds and use Bitcoin. Taxes might not be fair enough and probably this will happen.