The problem with handhelds seems to be FM immunity
That may be true but it is the first time I have heard that in connection with a handheld. FM Immunity refers to receiving equipment (VOR or LOC), not transmitting equipment.
Frankly if I was in need of "radio" assistance I'd use my mobile phone
There are 2 issues with this.
1) The reason for carrying a VHF handheld is that it gives you a seamless comms capability in the event of a total electrical failure. Obviously you need a headset adaptor for it, but I know that mine is good for at least 10-20 miles and if connected to a normal roof mounted VHF aerial (using a piece of BNC coax) it is almost as good as the built in radio.
2) GSM does not usually work above about 2000ft, and who will you be phoning anyway? Do you know the mobile # of the person on the current D&D or your local airport shift? Even the rescue centre in Falmouth do not publish a mobile number to which one could text a GPS position derived from a satellite phone, which I think is really dim (told them so ages ago) since every satphone can do this.