I have 2 that were butts to draw/write.
Believe it or not, the "Pestilence" arc was started early, around 3 years ago, and I burned out halfway through and didn't draw for 6 months. Luckily I wasn't online so I didn't have the obligation to stay on a schedule. It wasn't until last year, when my buffer was almost up, when I finally started again and finished it.
And there's "The Blind Owls", which was notorious to me because I was drawing around 30 panels almost every week to get through the arc. And guess what? I burned out HARD XD
Still, looking back I do appreciate the work I had to put in, and all that pain was worth it in the end.
(There will be more suffering in this next season, knowing me...but all will be worth it, for the story...)