Struggling for a few years with some questionable games? When? Unity was the only real flop and that was 1 year. Syndicate came back and proved that the series was better than a rushed game (albeit it was still out a year later so "rushed" to me is relative to the generation launch). Black flag was arguably the best in the series but didn't fully harness the new gen, then Unity a year later which was only current gen, then Syndicate (which is my personal favorite) and in the time it took for them to release that, they fixed the majority of the problems with Unity. Not to mention that at the same time as Unity, Rogue was launched on previous gen and it was a fantastic story. After Syndicate, they took a break from releasing something new and opted for remastering the Ezio collection followed up the next year by Origins which was the biggest overall game in the franchise up to that point, and immediately after that by the remaster of Rogue for current gen. In this whole time frame, I don't see where they had anything horrible, other than the botched launch of Unity which was fixed moderately quickly.
I get that I'm coming off as a fanboy for the series but I really do like it and it's nice to see people give it praise, but I also try to be factually correct and the statement "a few years" isn't accurate as well as it's misleading to say they have "questionable" games. Every one of them has a decent, if not an amazing story and the only things that people take issue with is how glitchy and buggy Unity was at launch, which has been resolved already so it shouldn't still carry the same kind of stigma as it did originally. That kind of focus on the negativity isn't appropriate for the series as a whole, especially when it was one instance in between 2 highly praised games and it was released 4 years ago. Apologies for going on a rant, but to me it's not fitting to say that about this series in particular.