you are almost never wrong fitting a DCU but I feel the bonuses are too small to be worth it on most mission ships. I’d rather fit something else.
sure you can passive tank a BS but the resulting tank is usually kinda meh, and uses up a ton of slots. Rattlesnake is one of the few that pulls it off well if you really want to do that though.
hardeners create a ton of tank, +50%+ resists is going to give more tank than just about anything else especially when you know what you are up against and can fit multiple hardeners for that. 1 booster with 2 hardeners is going to be a way better tank than trying to fit 3 boosters. Sure I can just throw 2 invluns on most ships and call it good enough, but I mostly fly high dps gank ships that can deal with it.
resist rigs are inflexible, it’s another one of those I’d rather just go with something better all around. For things like burner mission ships sure it can make sense as you are fitting the ship to fight one enemy, but for general mission running I wouldn’t.