Had a guy bring one of the new P-51 trainers to the field the other day. He had his own 4 ch buddy box. Problem was that every time we pushed the trainer button the flaps would deploy (servo installed obviously). No flap control on the buddy box. We took the easy way out and retracted the flaps and disconnected the servo. So I guess if the buddy box does not have enough channels for the installed equipment you MAY have a problem. THIS is one of the reasons for a thorough preflight before starting the engine.