A significant drawback I've found (I've got one of their hybrids) is reduced options for mounting things:
* My toddler seat has to go lower than it otherwise would, making it much harder to fit panniers underneath.
* I can't fit a triangular tool bag there
* It makes fitting a D-lock mount inside the triangle harder
Also it makes shouldering the bike harder -- your shoulder is further forward meaning the bike isn't as well balanced.