Yes
No
I use them regularly. Mostly the crossbow and rocket launcher as I mostly play skills builds. The rocket launcher is OK against dogs and gun trolleys but of next to no use at all against a heavy. Same goes for the crossbow really. They're OK but not so great that I see them as essential. The crossbow has undergone several changes already, with one period designing them around cracking armour that wasn't all that successful either. They need more damage to be effective.
I use the grenade launcher or crossbow against war hounds and unarmored elites. Otherwise, generally shoot things. Go lots of missions without use it.
But generally I use strategic placement of my turret for most engagements. Started using Artillery turret and that does more damage against armored elites than any signature weapon. At least I see bleed damage and slow them to hit them with gunfire.
Hardly use it because of lack of ammo. Usually when i remember i have it, i switch to it and have no ammo. The current system of making kills a certain way to get ammo is clunky and you dont remember to use it. If you picked up ammo from enemies that had sig weapon ammo you might use it more, but the special requirement to make sig weapon ammo drop is weird.
Side note - im a gunner spec pretty regularly - why would i use the minigun when the gunner spec gives 50 ammo bag, as the lmg im using is basically a minigun but i can use it in cover? Why would you spec into sharpshooter to get another sniper rifle as a spec weapon? It feels weird to me that a lmg spec tree, gets a bigger lmg as a spec weapon, and a sniper tree gets a bigger sniper as a spec weapon. The crossbow is a weapon unique to the class, the flamer, the missle launcher, the grenade launcher etc. It feels a little niche? Your into lmgs, so you get ANOTHER lmg type weapon.
Id prefer if the gunner talent was a deployable turret / minigun - like teh shield wall thingy from Div1 - THEN id use it all the time - specially if you were bulletproof for a little bit from the front.
TLDR - needs more ammo drops to remember to use it / needs faster deploy times / needs unique feature to make it stand out as a special "oh sheeet here comes the pain" type weapon for emergencies? (what are these unique class specific weps FOR??)