(Spoilers)Mass Effect 2 ruined the Franchise, 3 was continuation of that

[Olinser]

The main problem that people had with the ending was simple - they were promised that the choices that they had made throughout the entire trilogy would affect the ending.

They did not.

You could have played ultra-Renegade and exterminated the Rachni, sacrificed the Council and formed a human council, wiped out the Thorian colonists, killed Wrex, killed the Geth heretics, and yet you are given the EXACT SAME OPTIONS as somebody who allowed the Rachni to survive, saved the original Council, spared the Thorian colonists, and rewrote the Geth heretics (those are just off the top of my head to Mass Effect 2, but that list could go on for a long time, as I'm sure everybody is aware).

That is what made a lot of fans angry (myself included). For 2 full games I'd actually gone to the trouble of keeping 2 full game completion saves with Pure Paragon/Renegade choices maintained throughout, thinking that they might be necessary to access the final endings for each path. It was utterly pointless.

NOTHING you do matters or changes the endings. The endings are flat and fixed. The Mass Relays explode, incidentally devastating every major world actually visited throughout the games and presumably killing just about every major character not on the Normandy. No choice you make actually changes anything, and what really set players off was they were too lazy to even make actual distinct cutscenes for each one.

That's the problem at its core. BioWare promised fans something they pretty unambiguously did not deliver. And you can still think that the endings were good (I thought a couple were decent ideas but utterly refuse to accept that there was no option to reject the core premise that AI are inherently evil), while acknowledging that BioWare did not keep their promise.