A lot of people hated it because a lot of hype was built up before ME3 was released (kinda like the Destiny hype) about how there were going to be several different endings based on choices you made throughout the entire trilogy, but in reality the ending choices were just "red, green, or blue" and the ending cutscene was exactly the same regardless of what you chose. A lot of people, including myself, were pissed off about it because we felt like we had been lied to by BioWare, and we felt that the publisher, EA, probably rushed them to finish their game which meant that the writers had to give up artistic integrity just to put together an ending. There were so-called apologists who tried to defend BioWare back then as well, trying to argue that the complainers were just a vocal minority but that simply wasn't true. I only continued playing ME3 for the multiplayer, which was damn good, by the way. The multiplayer saved that game, in my opinion, and it helped that BioWare used their microtransaction money to make multiplayer DLC packs that they shipped for free, so I never had to pay an extra cent to continue enjoying a game that I think they messed up big on. The similarity between the BioWare/EA and Bungie/Activision relationships is staggering, though.