I think the fact that Zoro and Luffy are both getting power boosts through haki, as opposed to nazi science pretty much says it all.
The fact that they are using immense amounts of haki to increase their strength through Enma, or through G4 suggests that their power ups are strong enough that they need to be restricted with haki time limits.
Cook on the other hand simply got an all-round stat boost. This suggests that there was so much wrong Oda just had to give him a power up that fixed everything.
The fact that it's something he can continuously use shows that it's nothing too strong that it needs to be restricted. Just a tool to pull Sanji up to the respectable level that Zoro was once on, so that Zoro can excel even higher.