Local color lab mounts polypropylene ultra glossy prints on plexiglass with pressure sensitive adhesive.
I have read a bunch of stuff about mounting substrate surface tension and orange peel effect and I think it is all a load of krap. Once your eye is attuned to seeing it, it'll drive you nuts. I tried mounting photos on plexiglass with a couple different drymount tissue adhesives and they still had orange peel. So I think it's hype being pushed by the mounting tissue companies. I think it's in the adhesive....or worse yet, if it's an artifact of constraining the photo substrate, maybe dry mounting has an inherent 'look', better than soe things, worse than others..
One potential problem with foamcore is mounting on one side leaves the other side free to bow. There are procedures around for mounting plain paper on the opposite side to counter this, but the trouble you might as well use a (more rigid) board. For large things that aren't going to be framed, we use Gatorfoam because it doesn't bow, but on photos I don't hink it looks any better.
We've been mounting on matboard with Colormount as the most practical compromise and I don't like the appearance of anything other than the pressure-sensitive adhesive. So I personally stay away from the mouting so I don't get bent out of shape over it, the employees handle it, customers don't complain, but I swear I see orange peel when I look (no, it's not my glasses).
Maybe it really is the attunement thing - we have a designer who will not use TruView Conservation Clear glass because she can see the UV filter coating (I can see it in the right light if I look for it).
We have a huge expensive dry mount press that ain't broke, so my wife is't interested in switching to the 3M pressure mount stuff for equipment reasons. Not sure if it could replace our press in terms of capacity (44"x68"), and I imagine there is another class of challenges with pressure mounting (dust, debris).
Humidity comment from Bob C - some mounting materials advise pre-drying (in a press before repeating with the mounting materials), but I think that refers to the non-vacuum type press (looks more like a clothes press). The manual for our vacuum press says pre-drying needn't be done and doesn't mention humidity.