First I want to clarify that I am not talking about the invisible pore sealers that are used to seal tile and grout. Those are silicone-based as well, but my post about mosaic birdhouses was talking about using silicone caulks and sealing gels as an adhesive for mounting tile. I have serious doubts about these.
I have seen some silicone adhesives used on tile sample boards that was so strong that it was impossible to pry off a tile without breaking it, but the tile did come off in pieces. However, with fully cured thinset mortar or Weldbond (white PVA adhesive), the tiles sometimes have to be chiseled off into a powder in order to repair a mosaic.
This may seem like a meaningless distinction if the silicone is strong enough, but over time the difference in strength will show itself in missing tiles.
Also, how well does the silicone product age? Does it become brittle? What else is in it? Will the flexibility of the silicone gel or caulk allow the tile to move and crack up the grout over time? Will it be like the old caulk or old silicone gel on a piece of window glass, capable of being scraped off fairly cleanly?
I’ve done some follow up research, and learned a couple of things. First, a silicone adhesive or caulk may be a good option for people making mosaic mannequins or mosaics on plastic or fiberglass sculptures. Note that I haven’t evaluated this myself.
Another thing that I have learned is that a silicone-based caulk should not get brittle in ultraviolet light or cold temperatures in the same way that acrylic-based caulks will. Silicone is also waterproof.
However, the flexibility of silicone-based products when fully cured is still a problem in my opinion for mounting architectural mosaics. If the grout is to remain intact (and thus protect the substrate from water damage), then the tiles cannot be moving slightly every time pressure is applied to the surface of the mosaic.