Something came to my mind when i was thinking about the upcomming relaunch of several titles under a continuation of the current status quo.
Claremeonts efforts on the X-men over more than 15 years working on them, together with those working alongside him on the other titles, helped shaping the X-men and their offshot teams into a unique and specific form, that stands out and also attracts fans in a way that other super hero teams (or larger groups) can't as easily.
So popular were the X-men when he left that there is the half-joking opinion that they could have maintained their own publisher independent of Marvel in the early 90's.
However said shape also requires them to be written and maintained in a certain way in order to feel "right" to the longterm readers and fans, while also requiring constant updating and progression to keep the pace Claremont gave them. Which in turn seems to requires much more skill in writing their stories (short term and long term), more understanding of the characters, a good ability to balance out said characters in promince, knowledge of past events and stories and an understanding of balancing soap opera drama vs. action, than many other super hero teams or characters aswell (not to dismiss the others as less though).
Basicly they are much more high maintance and don't work well in too many different ways (while also providing a broad spectrum of stories ironicaly). Which might explain why it feels to me like the X-office not only often suffered from several mediocre writers over the years, but also somehow managed to make otherwise well regarded writers produce mediocre results. Because it seems working "the X-men way" is very difficult. Especialy when someone isn't as long on the title(s) as a decade.
Metaphorical speaking. The X-men as franchise are a powerfull and reliable car, which however needs a very specific set of tires to run well.
What i feel Claremeont's returns mostly suffer from in this regard is that he still keeps the old worn out tires around and slaps them back onto the car, but they aren't the right ones anymore either. It's not the past anymore, but it doesn't feel up to date anymore either. The car gets rolling, but the ride feels off.
And to make matters worse some have tried or enforced to reinvent the wheel completely over the past 2 decades damaging the car in the process.