IMHO "overrated" and "underrated" are definitely misused.

i use overrated when i find a (not just minor, but substantial/major) discrepancy between critical/popular acclaim (being very/extremely high) for something that is not necessarily terrible or bad, but just nowhere near THAT good.

most things are just ok or perhaps somewhat good etc. there is nothing wrong with that. average must exist for good to exist.

also, if an attraction is rated highly in a field, like architecture, then you have to be an architect etc in order to judge whether you think its overrated in that field. its faulty to suggest that everyone rates it highly for all reasons, because of one field.

OTOH, an underlying theme of these discussions may be the propensity for people to overrate things in general, although the degree of discrepancy is not always substantial. critics may also overrate when there is a style or message that is fashionable.

even without having read the other thread, i will bet there is some truth to the OP though - "most unfairly maligned" - because i will wager the negative comments are more often over the top than trying to do actual analysis.