I think Dexter and GOT are actually pretty similar in their drops, in that they had very uneven middle points(season 5) that could've gone either way, but they ultimately chose the worst paths, that basically abandoned good writing for the lowest common denominator. Dexter turned into "Super Dexter", where he was treated like a hero, and the whole show was all about him being a cool serial killer, with no thought put into anything. GOT turned into the "Zombies and Dragons" show, that completely abandoned it's intricate political drama roots.
Overall though, I cringed in embarrassment at damn near every episode of Dexter from Season 6-8. GOT at least had a halfway decent 6th season, and when I watched the rest, I wasn't mad, but more disappointed. The pieces were still there to flesh out. They just cut a LOT of corners. I was straight up pissed off at what the absolute shit show "Dexter" had become. It's like the show runners hated it, and wanted to make fun of it.