How a saddle feels on the trainer is different than how it will feel riding outside because we’re shifting our weight around alot more while road riding or mtb’ing. I think comfort in the saddle on the trainer for rides longer than 90 min is asking alot.
That being said, I have mountain bike saddles on my tri and road bikes. Apparently, my “sits bones” (ischial tuberosities) are spaced very wide apart–and here I thought it from all the Oreos I eat. The mtb saddles are wider and better support my caboose; and they are alot cheaper than tri saddles. Also, they are quite comfortable on the trainer. My longest trainer ride on them has been 4 hours and the only thing that went numb was my brain.