I can't remember what it's called, but there's a card that stops a player from moving a specific unit. It's to be played during the movement phase, which is not simultaneous, but that means if you are the last to go then the card is basically useless for that turn. Would it be better to allow it to be used at any point to stop another player's movement?

I would imagine it refers to the *entire* Movement Phase, not *your* Movement Phase, so the card is not useless.

If you're talking about the resource ability on Communism, I'm pretty sure the card specifies that it can be used to interrupt somebody else's movement.

And the culture cards that do the same thing also have that note as well.

I think it's worth noting that the communism ability prevents figures from leaving a designated space, which means that you can force multiple units to stay put.