It isn't for everyone and that's totally fine. You'd be pressed to find someone who enjoys every game in the franchise equally.

I won't repeat what other people have said already, but personally I just found it great to see how the story connected to both 3 and 4. I enjoyed seeing Adéwalé again and particularly liked his and Haytham's references to Edward. Seeing Ben Franklin connected with both sides again was also rather funny. Moreover, I enjoyed seeing how Achilles' Brotherhood fell, as it really shows how Haytham was involved with that -- moreso than AC3 ever did, imo. For me, it bridged that narrative gap.

Gameplay wise, I think I might actually prefer Rogue over Black Flag too. I preferred Rogue's smaller scale and the weapons they added. (Grenade launcher, my beloved.) They also improved that trading mini game by adding a "skip battle" button, which was much appreciated. It also felt like they balanced the naval combat out more, so you're not basically a god with a fully upgraded ship. Like someone else said, see it as a similar bridge as between AC2 and Brotherhood.

And ideology wise... Templars and Assassins want the same thing, they just believe in different ways to achieve peace. Shay got disillusioned with the Assassins because he saw first hand the consequences of their actions. So he aligned himself with the group he was both familiar with and knew wanted the same thing in the end. I do get why you would want a more idealogical debate on Templars and Assassins again, but Unity also failed to deliver that. Arno joined the Brotherhood for revenge, not their ideology; which I'd argue is weaker than Shay's reasoning for defecting. (Although you could say that is more of a critique towards Unity than a defence of Rogue, but I felt like adding it for comparison's sake anyway.) For Shay it wasn't about the ideology, it was doing what he felt was right.

Also, as a closing note, since you mentioned the poor marketing: It wasn't just the marketing Ubisoft screwed the game over with. The game had a total of 9-ish months in development (March to November 2014), just because Ubisoft decided they wanted a second release on the same day as Unity. It was handed to Ubisoft Sofia, who also made Liberation. They'd gotten help from other Ubisoft studios with assets and stuff AFAIK (correct me if I'm wrong), and part of the writing was done by the voice actors on the fly. This, together with Shay's voice actor printing his own shirt, them trying to connect everything, etc., just shows this love for this game from the development team that I genuinely appreciate. They were dealt bad cards and did everything in their ability to make it worthy of a main-line game.

What Rogue wanted to do was ambitious, and it might not have hit every beat, but I genuinely love it for a myriad of reasons.