I think it's fine to allow an opponent to play outside his turn. I think it could very well be a pretty strong effect (for your opponent) though, so this card might be quite weak. If they had too many terminals in their hand, you could have just turned a bad turn into a good one. Especially if they had just a terminal draw and no villages. And then of course they could play one of these in response, which would be a little hard to track/follow.
Also remember, you don't just have "an opponent". You have to say "each other player may play an action card from his hand". So imagine both opponents respond by playing one of these.... then things get basically impossible to track.