IMHO, most manga artists fxxk up in long series...
I agree.
But there are also people who can do things right. Like the author of Negima. The people who published his work wanted another harem manga like his previous work Love Hina, but he wanted to do a Magic/Fighting manga.
He was forced to do a harem.
In the end he won though. He genre shifted the whole thing into a magic/fighting with a harem of girls that kick ass with a plot that I can honestly say is very good.
Tell me, does that not sound awesome?