Unsustainable public finances, and a likely failure in the social safety net as demographics become unable to support the weight of the huge boomer cohort becoming recipients of benefits rather than contributors (bear in mind, in North America and Europe, boomers had less kids than their parents)?

A low growth economy for the past decade, following the Great Financial Crisis?
A job market that fetishizes higher education, resulting in crippling student debt, offset with low income growth and low security, all while housing prices run away from new buyers (with established, property-owning boomers largely happy to benefit)?

I'm not much for whining about it, but to ignore that the millennials got dealt a bad hand (while boomers benefited and kicked the can down the road) would be a mistake.