There are lots of adhesives that are suitable to fix to ply. The problem is with the ply itself, with most of the far eastern stuff delaminating and the glues used to make them reacting with the adhesive.
adhesive is mixed with water,this in itself is putting moisture onto the ply
bathrooms also hold heat when showers etc are on and lots of steam this imo can make ply warp especially on shower walls ive seen it a number of times
grout is also water resistant not proof another opportunity for water to ingress the ply
its just my personall choice but these are some reasons i dont like ply especialyl in bathrooms barry
hope this helps
also a lot of ply especially from b and q etc is sheeething off and is poor quality at the moment this could also cause a failure
if you overlay existing floor wiyhout taking up you dont know if there is any pipework that may fail in the near futur we have seen it all to do a proper job you need to know whats going on under the floor once tiled you dont want a problem under the floor i dont belive what i do is wrong i can tell the customer everything under the floor is perfect then 18 or sometimes25mm ply if you overboard and have not completle checked under floor dont think thats a better job