As a background, I loved the Ezio games and also enjoyed AC3 somewhat. I also love open world RPGs in general. But I hate grinding and mandatory generic side quests.
I tried it years ago, but did not like it and stopped playing after some hours. Assassinations via sneaking up and one-shotting were not possible AFAIR, which ruined the fun on assassinations for me. RPG mechanics like leveling and skills were present, but were designed in a way that added nothing of value to the experience while requiring a boring grind. There were many side quests, but they felt boring and generic and. I could have overlooked these things and concentrated on the main story, but engaging in the level grind and the generic side quests was to a large degree mandatory to be able to continue the story. That made me feel like I’m wasting my time and made me stop playing.
Overall I felt that the game tried to find some compromise between story-based action adventure and open-world RPG, but just ended up combining the worst of both worlds. It felt like the RPG features were pushed in top-down (“everyone is doing open world, levels and skill trees now, we should put that in the game”) without any regard to WHY these features work well in some games and how they have to be integrated in order to make the experience more fun.