The UPC (national plumbing code) requires the waterproof layer in a shower to be sloped towards the drain. Many contractors seem to feel they don't have to do that, and install a shower liner on the floor, flat. Water will get below the tile in a shower floor, so then, where can it go if the pan is flat on the floor? Nowhere...
There are numerous approved methods to building a shower. Recently, I did one for my mother. I used the Kerdi system from
www.schluter.com. It is pretty bulletproof if done right (as are any of the approved methods). There are too many horror stories about poor jobs. Most are done well, but more than you want to believe are done wrong. It is best to educate yourself a little so yo ucan sort out a compentant contractor, if you aren't going to do it yourself. Personally, I think a fiberglass shower pan is a waste of money...then don't look that great new, and look worse as they age and the gloss starts to go and microfractures start to occur.
Any shower system can give you problems if it isn't installed properly...a good tile job can last you for life...you'd probably tire of it's style, move, or die before it failed. A poor job can start giving you problems as soon as it is built.
If you want to do some homework, check out
www.johnbridge.com site "Tile Your World" for some inspiration and help.