Ceiling joists may not be strong enough for the concentrated load unless you put it over a load bearing wall. The bladder probably won't last as long in the heat. You may not like the hot water stored in the tank. Not sure why you'd need a T&P valve on it, but I'm not a pro, either, just giving you my thoughts.
When I lived in Kuwait, we had a water tank on the roof, not enclosed - it was white fiberglass. In the hot season (about 8 months of the year!), we had to turn off the water heater and use the cold water from the roof for hot, it got that hot. God help you if you ran through the cold water in the tank inside otherwise, you'd end up wiht hot and hot on both sides. It got confusing trying to remember which way to get hot, especially for a few weeks when the stuff on the roof wasn't that hot, and you didn't want to turn the water heater back on yet when it got to what they call winter there!