Wouldn't use tile grout as a substrate alone...... The sealer required us pretty toxic for animals etc.......I think if you get 20 reptile owners in a room and ask about the best substrate, you will likely get 20 different answers.
I am planning on making a custom vivarium with a textured floor that can be cleaned easily. I believe you can use tile grout and then paint over it for a natural look. Anything that gives a bit of grip, but can be easily cleaned, washed/disinfected without leaving loose bits has got to be good surely? Again, keep an eye on temps though.