To add to this, the Sentinel has many defensive tricks, more than any other kit really. It's very hard to get it's defensive abilities doing consistent damage. (They can do damage though. It's just difficult and requires a bit of luck)
The PPR by itself does so much damage that it counteracts the Sentinel's lack. The PPR does the most damage when it's ramped up, and the volus double cloak, decoy, and drone really help that by mitigating staggers.
The combat drone does a bit of damage with the right evolutions (but it's still miles behind powers like Reave or Dark Channel). It's good against Geth as a distraction, and Phantoms specifically. All other enemies more or less ignore it, same goes for the Decoy.
I find other weapons to be better for him, especially guns like the Harrier. The Volus Merc need to spam his powers frequently which don't synergize that well with the PPR. It's a great gun for sure, but the ramp up mechanic is annoying when you want to use powers frequently.