* You have to model explicitly the model walls. That is you can't just say "using X material, print this model with whatever minimum thickness is necessary".
* Even if the model visually looks correct, you may have to remove hidden surfaces or non-continuous (or connected) surfaces.
* Even if an online service like shapeways verifies a model it may not print correctly. And it's not always clear how to fix it.
* Solid models are extremely expensive and so you have to create a shell. Shells have several issues like print orientation, holes for the excess material to drain out, thickness of solid, overhangs, etc edges.
* Typically shapeways, etc either don't support certain formats or don't support them well. This means you have to convert your model into whatever format they support. This can introduce more errors.
* Automatically re-sizing - strangely enough if you can print a say 10cm tall model on a printer you can't necessarily print a 15cm version, even if it fits in the print volume.
Given all these things, even if you have a decent model, there is a large subsequent step in turning that model into something that can print on a specific printer and material. It's likely that printer hardware makers have to create some software that takes popular native model formats like .stl, .blender, etc and convert it reliably into a printable model for that specific printer/material.