Well the downside is they would lose a tiny insignificant amount of profit as they would no longer be allowed to abuse their workforce quite so much. If they actually sat down and did the maths they would work out that it isn't really worth fighting against it. But a lot of Republicans don't like it on principle.

That's why they always come up with hand wavy arguments about the need to incentivise work. You can always see them a mile off because they're the type that keep yelling about how minimum wage should not be a livable wage, because it discourages people to advance.

I'm assuming you want to abolish money and free markets altogether, because otherwise we're just talking about capitalism with social safety nets a la most of Europe.

So: Depends. Are we making working voluntary, or are we forcing everyone into jobs they may or may not like?

If it's the former, then there's no reason to work jobs that don't seem interesting or fulfilling. Good luck keeping the garbage service running.

If it's the latter, then well, economically everything's great, but the downside is that people will be forced to do jobs they don't like.

If you DON'T want to abolish money and free markets altogether, then there's still going to be private property, so then that's the downside. It's the exact same one we already have from capitalism. Just less of it, because more capital is redistributed via taxation.

It's hard to distil into three words, so it fails to win elections.

Non-ironically: the fact that it's not liked by groups of people capable of sabotaging it. For example, is the USA at fault for the embargo on Cuba and all the hardships it goes through? Yes. Is it still a problem for people living in Cuba? Also yes.

Unfortunately, some things may not be your fault, but they are your problem. And as long as there will be entities who don't like you and are able to make your life worse (so basically forever as long as there exist powerful non socialist nations), that's a problem you have to deal with.

And I don't say this as a critique of socialism itself, yet it's still a downside of it. Unfortunately, being liked by the bullies is an advantage and not being liked by the bullies is a downside, since they will bully you. And at the level of nations, there isn't a bigger authority you can go to. It's just school kids with bullies but there is no supervisor teacher. You could make the bullies unable to bully you by either becoming stronger then them or banding together with others who are against the bullies. But just like in a street fight, playing dirty is an advantage, so the bullies are at an advantage.

Again, this is not the fault of socialism, but it is one of its problems.

One name: Stalin!