My primary issue with McMansions specifically (beyond the fact that they lost the thread on western architecture, which should be blamed on the academy[2]) is that the materials and workmanship are terrible: ugly gaps, quick to stain stuccos and metals, slapdash construction and very little craftsmanship. The flip culture that the mortgage-debt bubble of the last 15 years created has exacerbated this issue to almost comical levels.
[1] - https://www.amazon.com/Old-Way-Seeing-Architecture-Magic/dp/...
[2] - https://www.amazon.com/Bauhaus-Our-House-Tom-Wolfe/dp/031242...
EDIT: After reading another post[a] of his, it is worth mentioning another chronic problem with modern (again chronological, not stylistic) building: the buildings often look like they are about to fall over. A particular pet peeve of mine is the flashing gap found at the base of many houses and buildings, which introduces a disconcerting negative gap right where a soothing, wide foundation should be. Visual insanity.
[a] - http://mcmansionhell.tumblr.com/post/148935246684/mcmansions...