I haven't bought many games that I have actively disliked in recent memory, not since I gave up on consoles and went to PC gaming exclusively. Even a lot of games people have mentioned, DA2 and ME3, have not ended up on my flop list. I actually liked DA2, I recognize a lot of its faults, but I did enjoy playing the game though I haven't touched it since DA3 came out. I also really liked ME3, right up to the ending. I admit the ending was total trash and broke some much established logic of characters and the setting. The extended cut DLC made it palatable for me to the point that I can reply the game to conclusion whenever I get the yearly 'play the ME series' itch.
I played MoO3 though I bought it second hand a few years after it came out when I was on the hunt for a good space 4X game. I felt let down by it. I didn't hate it, more disappointed. It felt like it had a lot of good ideas, but the execution just felt lacking. My biggest complaint I can remember is never being able to gain 'momentum', I always felt like I was struggling to get anywhere.
Will Stellaris be the big next flop for me personally? No. I have read too much in the Dev Diaries and interviews and I've watched all the streams and other videos. If this game is deeply flawed then Paradox is going to insane lengths to cover it up. My only concerns are getting used to controls which could be problematic. This is a stretch for me since I've played both CK2 and EU4 heavilly and I've never had issues with the controls. The other concern is how fast a game plays. In the Blorg streams we are just passed a century of in game time and they are firmly in the mid game. I was hoping each stage of the game (early, mid, late) would take three or four hundred years to reach. I like long games myself. But these are very minor issues. My experience with PDX games tells me that I will likely play the hell out of Stellaris for years to come and it will only get better as time goes on and DLC adds to the game itself.