Unity is really good, contrary to its reputation (at least on PC). The parkour ~~flies~~ flows nicely and doesn't suffer from the grappling hook bypass of Syndicate, it's still the best-looking game in the series, there's a lot of attention to detail and the story doesn't overstay its welcome, even if it sometimes annoys with time jumps and the series habit of having to cram in historic events that you already mentioned. Yes, the map is overflowing with icons, but at that point in the series, that's to be expected.

Compared to Black Flag, parkour, combat, stealth (especially stealth), presentation, architecture and NPCs are vastly superior, but it's less whimsical and of course doesn't have those fun (if repetitive and far too easy, unless you go for the big ships right in the beginning) arcadey ship battles and boarding attacks. Black Flag is definitely better at hiding and spreading its collectibles out, making discovering them a bit more of an occasion instead of an (admittedly still highly addictive) checklist chore due to its larger and more varied game world as well as the novelty of being able to leave your ship at any time, even in the middle of the ocean or near a tiny sand bank to go out and explore.