A few years ago when I installed my new Furuno radar system, I researched backstay-mounted fittings pretty thoroughly, including calling manufacturers, talking to others, reading everything I could find, etc.
At that time, I determined that this was NOT the way to go, since the available alternatives were expensive, unreliable, badly supported, and prone to failure. Also, I never liked the idea of attaching something like a 24" radome to my backstay. Decided in the end to go with a Kato pole on the stern, and have been very happy with that.
Things may have changed in the interim (about 4 years), but if I were re-doing it today I'd definitely again opt for the Kato pole.