They were fine when they were mostly just following the existing books, one book per season (more or less), and not having to come up with their own material or pacing.
Once they hit the end of the books, they completely lost all ability to pace (possibly because they'd never had to DO it before), so they wound up doing 2-3 seasons worth of just dicking around and accomplishing nothing of substance - until they suddenly realized they had to end things, so they rushed to an ending they put almost no effort into actually justifying.
Things might have been better if they'd actually taken the full season HBO offered them instead of the half-assed half-season they wanted so they could get it over with and move on to the huge money deal Disney was offering them to make a Star Wars series (which fell through anyway), but honestly, they were so inept at the seasons they DID do that it probably would have been terrible regardless of whether or not they had a full season, three full seasons, or a dozen full seasons more.
They always talked about how George RR Martin told them how the story was supposed to end, so all they had to do was bridge the gap between where the books ended and the end of the show themselves, but what no one seems to have realized is that they were completely incapable of actually doing it.
Ironically, they probably would have been better off if they'd just ignored the intended ending and gone with one of their own where Dany becomes the Queen in the South while Jon becomes King in the North or something. With Tyrion as some sort of ambassador/advisor between the two, because those are the only three characters most of the audience ever cared about anyway.
Yeah I think the fact that they were attempting to continue and end a story in such a short span of that a genius writer carefully took years to craft is something people overlooked hoping it would work out somehow. It was doomed to be disappointing from the beginning so they threw together a story and shoehorned in some mostly disappointing fan service hoping that would be enough