I expect the tech grenade to strip a Phantom's barrier and the homing grenade to put a dent in a Banshee's armour.
They do—very well, in fact... You're speccin' the Arc/Homing Grenades for Electrical/Fire Damage & Shield Overload/Armor Damage, right?
I think it may just be a matter of timing. In the case of Phantoms & Banshees, they both occasionally whip out these shields/barriers that practically negate incoming powers, and grenades are easily defended against. So, you have to carefully time your grenade throws to strike them while they're in the middle of attacking other players, stunned, or flipping around cover/biotically jumping around the place.
Try her out again with this in mind. When it comes to weapons, the only concern I have with the Demolisher is that she's equipped with a relatively lightweight weapon that'll let me spam Supply Pylons to keep my grenades stocked, and that Incendiary/Disruptor/Cyro Ammo powers are applied to set up a ton of Tech Explosions. N7 Hurricane with Stability/High-Velocity mods has treated me very well in this regard.
That said, if you can't see yourself relying on her grenades, you
can get away with speccin' her class tree for weapon damage, and equipping her with a couple of heavy, high-damage weapons, as her Supply Pylon is the only power that actually relies on her cooldown... But you'd have to play a pretty stationary game, as you won't be able to move the Supply Pylon around as much, and she's pretty squishy. There are better characters to go to if you want to rely primarily on weapons with the occasional grenade.
