That's very strange...
They are programmed as a "harasser" unit, which means they should continue to attack anything you order them to, even if it runs away. I haven't ever encountered the problems you describe, while using them. In terms of damage, they should be very comparable to Skaven "Warplock Jezzail" weapon teams.
Perhaps there's interference from another mod?

These were great in auto resolves, but I couldn't get them to actually do much damage... Against Britonia, I only managed about 6 kills each with them; they would just occasionally stop firing.

Thank you!

Really love the concept here, of just using a single ranged unit and I think this one fills the role well