There is no short answer to this question so lets start with lose tile. Many things durring a tile installation can ruin what looks to be a great install on the surface. Tile popping off the wall is usually the result of poor preparation or poor craftsmanship. If the surface is not prepared properly the thinset will not be able to enter the pores and adhesion will not take place. You can experience tile coming loose in less than 6 months along with grout joints cracking and powdering out.  However this is minor to what can truly happen in an install gone wrong. Poor shower construction can result in water vapor traveling into the wall cavity where this vapor collects and forms condensation. This now stagnant water trapped inside the wall cavity can now migrate or fester. This creates a rotting situation that if left undiscovered can result in massive damages. In one instance a faulty linear drain was installed in a home shower system and created a slow leak. That water built up inside the floor joists and above their kitchen. For 18 months water collected and black mold grew and by the time it was discovered floor joists were as saturated as fallen timber in the woods. Most of the mess was covered in a black mold and required an extensive clean up and rebuild of not only the shower system, but half of their kitchen as well. Costs like this just 18 months later can be unbearable for most homeowners and these situations are easily avoided.