Progressivism is flawed for a few reasons, but the major reason is quite simple:

Progress for the tick isn't the same as progress for the dog.

There is no "humanity as a whole" when it comes to valuations, which are the things upon which both aesthetics and ethics supervene. Utilitarianism ("greatest good for the greatest number") is incoherent, because valuations (the only things out of which "utility" could come) are ordinal, and you can't tally up a series of rankings. So the first problem that comes out of this is that the "greatest good" is a priori incalculable.

Another problem is that most progressives are (a) consequentialist, and (b) egalitarian, but these two things are irreconcilable. Consequentialism says that what we care about are outcomes, and the fact is that outcomes are made better by favouring some people over others--e.g. for the betterment of humanity we should probably devote more time and energy to Einstein than we should to the kid with down syndrome. This means that the progressive has to walk back their position and say "OK, well we shouldn't all be equal, just sort of equal," which sounds pragmatic, but is still incoherent; their belief system doesn't even agree with itself. The most we can say without getting too jiggy with cognitive dissonance, is that if outcomes are truly what matter and we're all equal, that all people deserve some very minimal level of consideration. This is, however, a very weak form of progressivism.