The final mission was very disappointing, but I cannot agree that Inquisitions ending is as bad, let alone worse, than Mass Effect 3's. While the final battle was incredibly poor the scene in Skyhold where you speak to your companions/advisors afterwards was well done and felt like a legitimate conclusion to the game. It treats the characters with respect and concludes their stories nice and cleanly; it doesn't leave them stranded on a random planet in the middle of nowhere for no reason at all just so that it can shove a forced Adam and Eve parallel in our faces.
So no, It is nowhere near as bad as Mass Effect 3's ending in my eyes. That said, I do think that the final mission is even worse than Mass Effects final level, and I really didn't like Mass Effect 3's final level either. I expected a last-ditch huge assault on Skyhold where the keep would be severely damaged but thanks to all of the work we put in to building our Inquisition we would win this time, a thematic reversal of what happened at Haven.
But instead Coryphyfish radomly re-opens the breach and we go kill him completely alone on some floating rocks, with next to none of our troops or assets present. It felt incredibly rushed and unfinished, which was quite annoying given how bloated other aspects of the game were. Do we really need ten open world areas with dozens of fetch quests each when the final mission is so bare-bones?
Also, what is the point of rebuilding a massive, heavily defended fortress if it is never actually attacked?