I finished YuYuYu after about 4 months of delaying.
Spoiler I can see why people would hate that ending so much. Probably the most irksome thing about the entire show was how frequently they 180'd between having meaningful plot development and pure comedy. The ending really was deliberate in how it was going to drive that in home right when you're at the climax of the story. Though I will say, the troll-y bait and switches for comedy and story really did make the show, though I can see that it would be fairly ruining for some people.
The ending itself is ambiguous enough so they really can do anything from there on, including full slice of life if they wanted. No clue how the manga's fairing though. Apparently YuYuYu was also just an adaptation of a sequel manga to the original light novel, which covered Washio's story 2 years before the current events.