RPGM is easy to get into. RenPy isn't too hard either, but RPGM has a bunch of inbuilt functionality that anyone can pick up and use right out of the box, so to speak. Which explains the deluge of low quality projects. The engine gets a bad rep, because most people aren't familiar with how powerful it is, and how much freedom it gives the developer(s).
The only thing RPGM is really missing is less hardcoding in the stats and formulae department, because that takes a bit of javascripting to get to. But there are so many plugins out there (as already mentioned) that game quality comes down to developer effort (or lack thereof) and asset quality (art, music, writing, etc.).