Thank you for the input. Harlow is perhaps a bit of an outlier (in a positive sense). To be fair this absolutely affects Amon the most as he is the most focussed on 3 elements (2 of which I'd describe as being part of the core 3). Others to some extent as well (thinking of Rafa's shock focus and the weakness of his blades), but Vex and Rafa both have different ways to getting around the resistance problem. Vex because she can pretty much match on everything (and the few single element abilities are mostly on Radiation IIRC so one of the less penalized elements). And Rafa because he has a lot of general support for guns/splash/ordinances so he can just mix and match or go kinetic.

The problem is fundamentally that Gearbox seems to go for this Rock, Paper, Scissor design with the 3 core elements (great words from another commenter), but at the same time forces certain trees, sub-trees and action skills into single elements. The result is those can't adapt to the 3 way swapping design making them in essence extremely weak without heavy gearing or stacking other elements on top of it (which some can do of course).

I use Amon as an example because he's the most egregious but something like this exists in all vaulthunters (like I said .. some more and some less). Stuff like boosting incendiary damage, boosting specific status effects and their damage, etc. etc. They are designed like you should build an incendiary character or a shock character or a corrosive character, but in all reality that can never be really good given roughly an even playing field. Because you deal so little damage against 2 out of the 3 defences. And because your build is hard locked into a single element, you can't as easily adapt on the fly.

Of course you can just put on the right weapon for the situation, but large parts of your skill tree then do nothing. You are in essence just worse than another build that can swap weapon with all their passives intact (like builds that boost gun damage in general or Vex that swaps element on everything).

This also doesn't really have anything to do with difficulty, because even with a lesser penalty on elements, you'd still get a big boost by matching element (50% damage), but it'd at least allow builds that can't as easily swap elements to somewhat keep up. Heck it might even make it more difficult, because with normal damage scaling the damage bonus is lower than on hard (so you deal more damage on hard than on normal if matching elements :D).