D&D 5E Help Action? Most underrated rule?

The Find Familiar Help is not OP especially if charcoal, incense, and herbs in your game are very cheap. 10gp worth may be a large volume that can't easily be carried and require a very large brass brazier that can't just be carried around by a character without a Bag of Holding or some such container. It makes the cost of casting Find Familiar a heck of a lot more than just 10gp. When it's going to require going back to town or to your Wizard tower, etc. to be able to recast the spell, you're much less likely to put your Familiar at risk by having it get within reach of an enemy to grant the Help action on an attack or deliver a Touch spell, etc.

Mastermind 3/Chainlock 3 - Master of tactics and a sexy familiar. That's a lot of help w/o using an actual action.

Help in combat is fine IMO, and even a light amount of attention paid to swatting familiars should keep things in check. Out of combat? It need to pass the "uncle Jim being helpful" test. You know how that works, you're trying to do something fiddly and your helpful but useless relative, lets call him Uncle Jim, always has a "well, hey, let me help you with that...". If the action in question is one where you'd say "sure Jim, can you ..." then maybe help is appropriate. But if it's one of those actions where you'd politely tell Jim to piss off right back upstairs and out of your workshop, then help isn't an option.

For example, I don't generally allow help on lock picking. "Can I hold your pick while you twist?" No thanks. When it comes to it being game breaking, it isn't, even if happens on every flaming roll. Why? Because the DM sets the DC. So help away gents, I'll adjust DCs if I feel the need.

Note for the people suggesting the opponent can just "Swat that owl out of the sky": Owls do not provoke attacks of opportunity, have a high movement rate, and can fly. There is no onus on them to stay in combat with their target. So they can swoop in, grant advantage, and swoop out.