Shiro's last minute gay wedding to some rando as the closing shot is the most infamous, but the problems with the writing go way beyond that.
There is a subplot that lasts about a third of the season where Allura invites a random, unexplained "dark entity" into her head, against the urging of her team, which leads them to Honerva's mind where they meet the trapped souls of the former Voltron paladins (including Zarkon and Alfor, who were not killed by Honerva) for...some reason. This subplot goes absolutely nowhere, there are no consequences for this, and it's never discussed after the team meet the old paladins.
Most egregiously, after spending the better part of the series setting up an arc for Allura about accepting she still has a family in Coran and the Voltron paladins, the show has her sacrifice herself to go be happy in the afterlife with her dead family AND the Space Elf Hitler who put the last survivors of her people in concentration camps and was manipulating her from day one. Lance retires to go be a sad farmer.
As for the action, I can only imagine the runoff from /a/ like >>115110282
like the over the top battles at the ending because it goes full Gurren Lagann at the end with Honerva literally destroying EVERY ALTERNATE UNIVERSE EVER one by one until Allura shows her some happy memories and then sacrifices her live to save the universe, because Allura has whatever powers the show needs her to have. (Contrary to his...bizarre opinion, "humanity" has nothing to do with saving the universe. It's all Allura. She just happens to have the power to un-erase all the universes Honerva just erased. Voltron jobs in basically every fight after Season 6.)
It was a lazy, uninspired ending that seemed to have a much different idea of the kind of show it was supposed to conclude.