Despite the whining to the contrary, yeah, scrappers actually have very high sustainable single-target DPS. Our AoE DPS is a bit sad, but that's not your job. Your job is to kill that boss before it enrages.
The key to sustainable DPS is to keep your buffs/debuffs up and your big attacks (backblast, pistol whip, suckerpunch) on CD.In a normal boss fight, I only end up using flurry and thermal grenade after a knockback, and sometimes fill in with a quick shot after triggering pugnacity. Otherwise, you have nearly unlimited energy if you're careful, you've got 2 bleeds running (flechette and vital shot), you've got 30% extra armor pen, and you push out back to back high-damage attacks.
It's not a face-rolling class, but if you're good at your rotation and good at your resource management, a scrapper is an excellent PVE build.