I've actually bought all the GC games despite only enjoying them marginally(not really my cup of tea), so my perspective is a bit atypical to start with, but the game has issues.
It's predecessor is better, objectively, in all but two subjects, the combat system, and the graphics. The combat system has great potential, but is mostly a disaster thanks to the poor state of carriers. It possesses a truly superior modern engine, and runs beautifully, but lacks the polish and depth present in it's predecessor. The arguments against this are disturbingly vapid at times, despite an obviousness to the flaws that is both acknowledged and often already set to be solved.
I've never liked Blizzard's games, they're safe, boring affairs, hyped beyond reason despite completely lacking much in the way of creativity and uniqueness, but they do have polish, and that is what makes them so successful. If Warcraft, Starcraft and Diablo had released in similar states as this game did, Blizzard wouldn't exist and three staples of PC gaming would be obscure relics of an era dominated by more interesting games.
I'm largely ambivalent to the lackluster state in which these games are released, I'm already testing in advance to begin with, so for me it's just an arbitrary date with zero meaning, but that doesn't mean it's just fine to release in a state that would leave pretty much everyone comparing it unfavorable to it's predecessor. It's a stupid business decision, one that probably cost them hundreds of thousands of sales. There's no excuse for Stardock to be hitting the 60's on Metacritic. They should be doing solid 80 plus games, they're well funded, self published, and digital only, so they lack the necessities of premature releases so often attributed to the downfall of companies.
GC3 is in no way an Elemental fuckup, but the decisions on when to release things leave me wondering they bother to put so much work into something only to have it hammered by poor reception for literally no reason. If it released today, it would be in the 80's, a solid beginning with a few design issues to work on further and some minor bugs and UI gripes, instead it's languishing in the 60's and 70's, with only the professional reviewers having given it largely undeserved ratings it will eventually far surpass.
When people say your new game isn't as good as it's predecessor, you can't tell them to wait a couple sequels for it to catch up because it's a new engine. If you have to work twice as hard to get it done, you put twice the time into it and wait for the sequel to start rolling in the money. A thousand hour player panned it yesterday, after replying to one of the less inspiring posts, shitting outside the doors to scare away your customers. How much money is lost on just that one bad review? I expect it's in the thousands.