Except for the fact virtually all have serious usability issues, bugs, and general problems.

The community spreads its efforts thin into a thousand projects rather than making one amazing one.

Windows and Mac also have usability issues and they are driven by two companies that don't allow break ups of their DEs.

I don't think that a FOSS community will ever converge on an amazing DE without part of it breaking apart and starting a new one, for the sake of building something even more amazing or failing on the way.