MOSFET reliability for critical applications
I have some background in high reliability electronics for space applications. The way you define "reliability" here is the key.

If you're thinking just in terms of the the random failure rate, a properly derated and protected flight-qualified MOSFET-based assembly can easily beat any equivalent relay.

But that's not the only thing to consider when chosing between those two technologies, of course. That would be too easy. :)


Are there any specific derating or reliable use guidelines for MOSFETs in aeronautical applications that make them unconvenient for your company? Has this technology been used before for this very purpose?
Are there any intrinsic advantages of one technology over the other? Such as higher immunity to single events (like lightnings, atmosferic radiation, etc.). Is there the possibility of single-event latch-ups (i.e. destructive response to an environmentally possible event) in any of those technologies?
Are there any environmental parameters that impacts more to one technology than the other (vibration, shocks, temperature, thermally induced mechanical stress, etc.) and that can precipitate accelerated aging and an early wear-out failure?
Which are the failure modes and their criticalities? Does any of the two alternatives have a significant advantage over it?


Sorry for raising more questions that answers, but your question can't be easily answered without having a broader view of the design problem.