Prefab shower stalls (one-piece molded fiberglass or multi-piece kits that include a prefab shower pan and wall and glass door panels) are easy to install and if properly installed will work just fine. These pre-fab units are also cheap. American Shower and Bath makes good units, and I’ve installed a lot of them.
The key to a good installation of these units is site preparation. The framing layout needs to be square, plumb, and level, and the drain and supply plumbing must coinicide with the manufacturer’s specifications.
The downside of using these pre-fabs is that if your layout does not accomodate the standard measurements (32″ and 36″ shower pans with exactly centered waste lines in the floor) you will have a difficult job to make them fit.
In such cases, the built-on-site mudded and tiled shower pan and shower wall approach will let you fit a shower into any space. Be aware that plumbing codes do require a minimum dimension of 30″x30″ (actually a 30″ diameter circle) for a newly constructed shower pan.
The disadvantage of a custom-built shower is the cost of the materials and the skill required to make it. The advantage is that it will fit any irregular space and plumbing layout, and, if you are an artisan, it will look very fine indeed.
NtP