I'm not one to read manga, I'm very very picky with what I read. But so far of all I've read, nine times out of ten, it has a very unsatisfying ending. Sometimes it feels like the serial was loosing popularity during it's run and the editors cut it suddenly, causing the artist to rush the wrap up. And sometimes it feels like the artist has no idea how to progress the story anymore and just straight up ran out of ideas and just ends it suddenly. The worse are the ones that set up something really big but then suddenly end it without really explaining or going into that big something. And a lot of these mangas tend to be very LONG. Meaning they go on and on and on until you're questioning if any real progress is going to be made to the main "mystery" and thenn... nope.
I do like the manga system in japan though, as to how comics work in the west. Over there, and correct me if I'm wrong, an editor will give a story/artist a shot, either for one issue or a small story arch, and if it's rated well they will be allowed to continue the manga. You get manga that last YEARS going on and on and some which just end after a handful of issues. Because of supply and demand. It kinda feels like in the west, with Marvel and DC, that sort of competitiveness doesn't quite exist. But at the same time, this system of creating stories causes me to read great mangas with disappointing endings. Here's what I've read to show you what I mean:
Blame!, Billy Bat, I am a Hero, Homunculus (wow I thought I read more than that, am I missing some? I guess it feels like a lot because there's so LONG)
The manga industry tends to cycle out things that drop in popularity. They usually provide some, but minimal, closure so they can get a new project rolling. I'm not nearly as prolific in comic and manga reading as with books, but I've seen it enough times to know it happens often. I don't have the stomach for a hundred issues of One Piece, which will certainly have a full conclusion if the author doesn't die first -- but that digression aside, it's usually better to find something that's already complete, or stick to 1-3 issue self-contained stories.
The benefit of the way the manga industry works as opposed to the studio comic industry is that new creators are entering the arena all the time, and largely control the content of their stories. Funnily enough, there was an anime a while back, I think it was called "Bakuman," that did a pretty good job of showing what it was like to work in manga as an artist, with some minor qualifications and exaggerations. Very publishing-house style of industry. Manga's a much better-oiled machine than the chronic gatekeeping that has stagnated Western comics. I must have tried for fifteen years to get into various Marvel and DC comics, but when it got to the point that every page had a little asterisk that was telling me to go read something else so I could understand what was happening, I had to cry foul on the writing.
Anime is actually a bit worse in regards to endings. I'm pretty sure the Index books have been finished for a while, but damned if there wasn't six years between the second and third season of the anime, after ending the second with way too many questions. So did the third, come to think of it...