The problem most people with tiled showers face is lack of waterproofing. Even in today's marketplace, where we have access to amazing waterproofing membranes and fabrics, many companies still use nothing.
Your tile is not waterproof, and neither is your grout. The thinset mortar behind it isn't a waterproofing membrane, and concrete board is about as waterproof as, well..., concrete. The truth is that when it comes to tile setting and building custom showers, we are still in the wild west. Building codes offer little help--only suggestions. At the end of the day, you are relying on your contractor.
Two tried and true methods are: RedGuard and Kerdi.
RedGuard is a liquid waterproofing membrane that gets applied to the shower bed, and or concrete board. Two or three layers are needed, with adequate dry times in between. However, proper installation will keep your house dry and your shower operational for a very very long time.
Kerdi is a fabric membrane made by Schluter that gets mortar'd into the shower pan and or concrete board. The company makes niches, corners, heated floor mats, and much more. This is the Cadillac of waterproofing, and they back it up with their warranty.
Regardless of which method you or your contractor chooses, make sure something stands between your shower and your wallet.