I have a Comar ASR 100 splitter used with an ICOM radio and stand-alone NASA AIS receiver and display. Both radio and AIS work just fine, no problems (as with some other posts on this topic).
Comar recommend that the splitter has the same power circuit as the radio. They also say "In the event of a fault or power failure to the ASR 100 the VHF Radio transmit and receive will continue as normal".
The Comar "allows your existing VHF antenna to be used by both a VHF radiotelephone and an AIS receiver" which I take to indicate that it cannot be used with an AIS transmitter.
I suspect, therefore, that where a problem exists, either the hardware or installation are at fault.
It also seems clear that Comar, at least, do not expect damage to the radio from failure of their splitter.
I hope this helps. The vexed question of whether a separate aerial is a better solution remains. With hindsight I might have gone for a second aerial, largely because it could also be used as an emergency radio antenna, also could still be used should I upgrade to an AIS sender. There does not appear to be a clear cost advantage either way.