They are a pain in the arse to set up properly and you really need a (edit: very patient, thank you for putting up with all my sine tones, Tommy) deaf person with a suitable hearing aid to help you set it up. If the loop is the wrong length and has the wrong impedance you'll never get the amp balanced up properly, and you can't just take the installer's word for it.
Beyond that I have very little experience of them.
Beyond that I have very little experience of them.