Sonik tec... I've not come across problems with hearing aids and synthesizers, but induction loops in churches and other public buildings can have issues with amplified instruments, particularly with reverb springs and magnetic pickups, also with unbalanced signal lines, and with dynamic microphones.

Unfortunately there are no guaranteed easy fixes because of the way reverb springs and pickups work, although using a DI box to balance the signal line usually sorts that problem. Dynamic microphones shouldn't really be used (although they can be okay if set up carefully)... electrets are recommended.

Many years ago I was called back to a theatre where we had installed a new loop system. They were having horrendous and unpredictable feedback problems... it turned out that someone had brought in an old Hammond organ (complete with reverb spring) and had connected it to the sound system without first checking that it was okay to do so... which isn't unreasonable, but undesirable!

It cost the theatre a call-out charge and mileage costs for a 150 mile round trip, though, as the equipment we had installed wasn't actually faulty.