Hmm. I just tried it against 3 Wardens - the ones that generally have the Thumper - and it has absolutely zero problem, but then those aren't too fast. Then I tried it against 3 Pickets - the fastest Derelict drones - and sometimes it gets them, sometimes it doesn't. The thing is, once they get close, it's hard to get the beams on target with manual control, too - they do a good job of strafing away, and the Sunder's angular momentum/relatively poor angular acceleration make it near impossible to keep up and stay on target.
When it's *not* difficult - i.e. when the drones are on the initial approach - I didn't see it having any trouble; it'd consistently nail one of them without any waffling. But once they get close... I mean, a Picket could basically just stay behind it 100% of the time if it really wanted to.