That's an excellant question and I have my own ideas about when/when not to use silicone adhesives for mounting objects in shadowboxes. I have no foundation to back up my ideas other than common sense so I will refrain from listing them here but I think that generally the use of silicone is limited to things like attaching foamcore spacers to float mounts (it works quite well when attaching things to suede board), attaching things that have no significant value to the owner, or using silicone for mounting things of a temporary nature/short lifespan. As you stated, there is harmful outgassing as silicone dries and you need to give it time to dry completely. As to any long term outgassing, I don't have a clue.
I would advise using some common sense when mounting with silicone and, when in doubt, use another mounting technique.
I would not use silicone with heirlooms (for obvious reasons) or objects of a valuable nature, any kind of fabric (the silicone will penetrate the fabric and become impossible to remove without damaging the fabric), hard objects with much indented detail (the silicone is difficult to clean out of deep seated details on medals and raised lettering/details on objects), and anything that you know would cause damage to the object either by chemical reaction or by future attempted removal of the adhesive.
The only time I was actually advised to use silicone adhesive was by the AIC back years ago on mounting collector plates into a shadowbox. They said that the silicone would stay flexible, not harm the glaze on the plates, and be more easily removed if required than any adhesive that would harden and have to be chiseled off or scraped off with an edged tool. They said that you could damage the glaze trying to remove a hard set adhesive where silicone could be more easily removed using solvents/tools that would not scratch the glaze. That was back in the early 90's and those recommendations may have changed as so many of the other recommendations of that era have changed and been updated.
Framerguy