Some of the best guns in the game are SMGs (sandhawk anyone?), and vladof pistols are usually good as well, first and foremost the anarchist (which can come in any element other than explosive).
Using elemental weapons is never, nor will it ever be pointless. Loaders rare? Are you sure you’re playing Borderlands 2? You’re fighting loaders, or armored enemies in at least 50% of the main campaign, and I’m not even gonna mention the DLCs.
Slag is an element, and if you’re in UVHM you should be doing this regardless of if you’re using other elemental weaponry, or not.
Armored psychos/maniacs/lunatics are only weak to corrosive damage in their chest area. Their head isn’t armored, and therefor is not weak to corrosive damage. If you wanted to test accurately you should have gone to Washburne Refinery where all enemies aside from Hyperius are loaders.