Sigh... why is it that authors have no clue how to end their series? I'm pretty sure like 80% of these series have bad endings. Is it really so hard to end it with them going out?
I think the issue is more related to how manga and serialization works in japan than authors being incompetent (though for some case, I do believe some lack of competence on the author part is also a factor, more on this later).
As most of us know, mangas runs in magazine for an undetermined time and are often subjected to reader's polling to measure how popular a series is. Which means that an author can pretty much runs a manga for a long time as long as it keep drawing a considerable interest (Yes, I'm looking at you, detective conan and your 1000+ chapters).
Based on that, unless an author actually want to go for a set number of chapters, they kinda have to keep things running while making some progress to the story to keep things entertaining, avoid readers getting bored and keep performing well enough to keep rolling. Reason why in romance mangas, the main couple is pretty much bound to keep avoiding confessing to each other because in a lot of people's mind : confession of main couple means nearing the end of the manga. So yeah, also why side couples are more lucky as they can make more progress without making it so it's about to end (room for two is actually a good example of this one, keeping the main girls in a sort of shraudinger yuri state while side couples are actually dating).
The consequence is that when a series is suddenly axed, the author is given a small number of chapter to actually wrap up the whole thing (usually around 3 to 5, it seems), with no regard of the current state of the series at the time.
So yeah, making a fitting end all of a sudden with just a few chapters to work with ? Probably means reworking a lot of things and that is, I believe, where an author talent will shine, as a skilled one might be able to do good under such dire circumstances, but at the same time, you might have been, prior to the axe falling, in such a state, story wise, that no amount of talent can credibly recover in time for the finish, which will way too often leave the reader with a feeling of discomfort.
Sorry for the wall of text, but thought it might help to share those thoughts. If not, well, just toss it in the bin o/