Yes. I got hired to integrate MLS data and man was that a nightmare. Many are surprisingly low-tech and almost all use different formats. There is a half-assed attempt at standards (RETS), but that's for accessing the data not the data itself. Not to mention getting and presenting the data legally--each MLS group has different rules about what you're allowed to display (and it can be incredibly restricting). tl;dr the project went down in flames.

I'd love to see Google get ahold of the data and make it available through Base.