No trapped air-bubbles. You have to make the tool yourself, which might be bit of an effort the first time. Cleaning the tool is more difficult. Unbeveled tile edges are a potential danger for scratching the mirror. Plaster or cement particles coming off the tool can cause scratches.
Dental plaster cures very fast, which makes it the best choice.
I made good experiences with white Porcelain epoxy. Of course it should be waterproof. Porcelain is best, because it is very hard. If you can only find big tiles, then cut them yourself with a tile cutter. Masking tape and aluminium foil.