It's not about the plot being nonsense (which it is), it's the harm it does to the world building and reality of the Federation and world of Star Trek "behind the scenes", or rather that off camera - this season presents a world of mass incompetence, on everyone's part, and takes away from the fictional world by shrinking it enormously.
Any comparison with The Undiscovered Country fails at that point as a send off, as that grew its world (not mercilessly cut it down), and equally with "What You Leave Behind" or "All Good Things". None of those finales fucked up the world by making everyone else stupid or having over the top villains (not even pah wraith Dukat, as we had a season and a half leading to that, and a long history of his own "believing his own bullshit and tragedy myth" going back to season 3 at least), nor have especially non-sensical plots, in pursuit of one final adventure with each crew.
Nor did any of them - even TUC, as our closest comparison, or AGT, with its seven year later flashbacks - use unwarranted nostalgia to tell their stories (or rather use nostalgia instead of a story). I think they actually expected their audiences to critically engage with them, to think the story had to be its best.
They work, and sadly season 3 of Picard doesn't work because as a mystery box it just couldn't tell the story it wanted to tell straight up, had to have (multiple) secret villains, and just be "wham bam don't think about it ma'am" as the only way to enjoy it.
Soooo frustrating, given what we could have had - namely a TUC that gives not only it's cast dignity*, but respected the world it depicted, and didn't take a series of cheap decisions designed to coast on your familiarity with the past greatest hits.
*(Except perhaps a certain scene with Uhura and language books)