I don't really mind their changes to Hotah, other than the fact that they introduced a character who is all about his giant axe and then never showed him fighting. My problems there are more about the complete lack of any characterization on Ellaria and the Sand Snakes. They're nothing but murder drones the whole season. The single best Sand Snake scene was the gratuitous Tyene topless scene which would usually be the sort of woman-takes-her-top-off-for-no-discernible-reason-that-we-must-have-at-least-twice-a-season scene that I would hate just because Nym and Obara rolling their eyes at her was the closest thing to a real human emotion they had all season. Add the hokey, poorly choreographed, whip-and-double-dagger combat on top of that, and it was all just bad. And then ending it on "Ok, we surrender... HAHA JK WE MURDERED HER ANYWAY" was just like.... what? And no master plan from Doran? Nothing?
I've said this before, but I think what frustrates me the most was how close they came to finding elegant solutions towards creating an abridged version of Feast/Dance, only to bungle them all. Sansa in Winterfell could have been a cool story where they transplant her political development that happens in the Vale in the books into the Northern story, and show her quietly rallying the Northern lords against the Boltons from within. They could have delayed the wedding long enough that it never happened and we never had to see Sansa raped. If they wanted the "warm her up, Reek" scene so badly, they could have used their Myranda character for it. Instead it's just non-stop Ramsay horror show, and all signs of Northern resistance are stomped out by Ramsay's almighty flaying knife.
Sending Jaime to Dorne would've worked fine to put a familiar face there and to delay the Riverlands plot to the next season, if they had just left the rest of the Dornish story largely intact instead of replacing it with four women yelling about murder. Even the Ellaria/Arianne merger could have worked fine, if they had actually included any of Arianne in her character. Instead of a story about someone foolishly thinking they can start a war and get everything they want without getting any blood on their hands at all only to have it all horribly fall apart, we get a story about how this lady wants to murder this girl, she gets stopped from murdering the girl, and then murders the girl anyway.
Doing Hardhome was a really smart way to inject some action into Jon's plot and to keep the White Walkers front and center, but then they just discard all logic after that to get to the stabbing. Alisser lets him in and lets the wildlings through, only to murder him the next night? Everyone saw the real threat, but that doesn't dissuade anyone from committing the murder? But it would be too hard to get to the Pink Letter to actually justify this stuff, so whatever, we'll just have it happen for no reason.
I think the only character where their changes really worked out was Tyrion. Cutting the Griffs to speed him along to Meereen didn't really end up harming his story much. I think Arya worked well, too, but that's because she got the pretty straightforward sort of adaptation that they've mostly done in previous seasons.
So yeah, anyway, I guess my point is that their greatest weakness is less in how they structure stories, whether they'd doing it over 7 or 8 seasons, and more the way they establish character and motivation on the micro-level when they don't already have it done for them by GRRM, and I can't really see that changing whether or not they have two seasons or three left.