That warlock fusion rifle is actually quite awesome. Naturally high stability that can be maxed out in exchange for a bit of a range loss, or it can keep its decent range for larger targets a bit farther out, plus it can cover all three damage types without having to use up three inventory slots & as far as other classes using an auto rifle shotgun combination: there are plenty of options in the game to facilitate that. I'd even wager there's options better than imobius and fabian strategy (which is likely why you'll rarely see a titan running around with both of those weapons equipped).