Failure to stick the landing has been a very common problem with very good series. Dexter, Scrubs, and Gilmore Girls comes to mind. Not that it's a "good" series, per se, as it was often hit or miss, but I was a diehard fan of Smallville and was incredibly disappointed in the last season and the finale, too. I've also never finished Battlestar Galactica's last season because I've heard it leaves a bad taste in people's mouths. And I never even picked up watching Lost because of its notoriously unsatisfying ending after its also notoriously addictive mystery breadcrumbs lead to nowhere. It applies to a few novel series from my childhood too, like the Pendragon series, and The Inheritance Cycle. It sucks when you devote years following a series, loving the characters, plots and mysteries, only to end up not ever wanting to consume it again because of the massive disappointment in the end.
Not sure Scrubs counts so much as gets an honorable mention, as Bill Lawrence himself even said that the last season was supposed to be a spin-off and not the actual last season. I never watched after JD left Sacred Heart.
Pendragon ends so poorly that I know few people who read the series as a kid and actually read the last book. The way you read the Pendragon series is by petering out in the last 2-3 books and just setting them down to never pick it up again.
Yes, I think writing a proper ending generally seems to be more difficult. Especially as a show gets more and more seasons. Which is why I think the opposite is kind of more interesting to discuss.
That's fair! I can think of shows where the beginning was meh, and they got better like Parks and Rec IMO, but I don't remember the ending being particularly amazing compared to the rest.