Constantly.
The Maximum Ride series is so guilty of this. James Patterson had a fun idea with loveable characters that should have made a great quartet, and instead he eviscerated it and dragged its entrails out for five more books that ended with the literal end of the world and then had the audacity to revive it for a sequel series. It wasn’t even fun by the end, it was exhausting. There were sixteen million unfinished plot threads, I didn’t care about any of the characters (and characters died and returned so often that it was impossible to be emotionally invested when one of them died/re-died), and Max and Angel were both insufferable.
Divergent kind of went off the rails, too. I actually like the trilogy but I can admit the worldbuilding was not all there and that was it’s downfall. Veronica Roth has another duology that I really enjoyed in the first book but ended up falling a part a little in the second. Roth apparently struggles with endings 
Shatter Me started out okay. Then the MC cried and made zero effort the entire second book, then decided she deserved to rule the entire continent of America because she was angry in the third book. Also for some reason Taherah Mafi tried to convince us that the first three books are a trilogy when not a single plot thread was properly concluded.
The 5th Wave, now that I’m thinking about it. That ending was awful.
Scrolling through my Goodreads, I want to add Infinity Son by Adam Silvera to this list because wtf was that. I normally like Silvera’s work but that one was bad.
Lightlark was decent but not fantastically well-written. Although I will say that the internet is going way too hard on how bad it is. Mob mentality, what can ye do.
Um, The Shards of Excalibur series is one I’m nearly finished. It’s weirdly not the only indie-published modern Arthurian fantasy I’m in the middle of right now. I like the plot and the pacing, and I don’t hate the characters, but the writing style is killing me. Everything is spelled out. The characters constantly think through what they’re doing, why they’re doing it, how it’s making them feel ect. It’s extra annoying because one of the pov characters is the antagonist, so we get to see exactly what his next move is going to be every step of the way and it kills any suspense. I’m hoping that this series is just particularly bad, because apparently Edward Willett is an award-winning author.
Okay, i’ve procrastinated enough, time to stop ranting for now 