I can understand sides not wanting a croakamancer, but we've seen them just as unwilling to capture a dirtamancer, predictamancer, and luckamancer.
In fact, the only side we've seen that was eager to capture casters is... Stanley.
Maybe he's not as much of an idiot as he seems, sometimes. Or capturing casters is a bad idea for reasons we haven't yet had discovered. (edit.. I said 'disclosed' lol, tired)
He did come up with the eyemancer linkup idea too. (As far as we know anyway)
For loyalty, I think turned units work like captured units (people, cities, planets, etc) in games where loyalty is a concern - they start off with a low loyalty and gradually build it up over time. Otherwise, captured units would always be essentially useless.
So you'd probably start off giving them nonessential but important tasks, make them feel like part of a side that appreciates them, cater to their strengths, etc, but also keep an eye on them and make sure they don't have a chance to screw you over, or to mitigate any damage they may do if it does happen.
I think a turnamancer just speeds up the process so they can be trusted sooner. So Jillian will be able to use and trust those captured units, and put them in positions where she won't have to worry about them wrecking her side or losing an essential battle on purpose.
A lot of this post is speculation. I'm not claiming it is how things actually work.