I'm done with this now, 130+ hours. So of course they just announced an updated adding a few things I would have really wanted (transmog, close camera)
River Raids were really disappointing, so much wasted potential. Raids are probably the best part of the game, being thematically cohesive and pitching yourself and your crew against monasteries, fortifications and camps. Putting aside the hand-waved justification for river raids alongside the game's plot of gathering allies, or the questions over how they fit (or don't fit) into the open world map, river raids promised a lot. Two of its main systems, of reactive river defenses and raided locations taking time to recover, are in principle very good. That's about as much as I can say for it.
I was unclear going in whether river raids were a full on procedurally generated game mode or a set of hand crafted self-contained river maps. Having played the mode it seems the developers were similarly confused. The rivers are hand-crafted and unchanging, yet almost all of the set locations along them are blatant copy and pastes of each other with repeated names like "small camp". The result is the worst of both worlds - it's like playing a roguelite but only ever being restricted to two specific seeds.
Wasted potential then, which is really how I would summarise the game as a whole. There is a lot I like, or want to like, and it obviously engaged me enough to stick with it over its insanely bloated playtime, but I'm just left wishing for a different game, one with strong direction and a creative vision (and one that knows when less is more). The biggest fault of Valhalla is its design by committee, and there are almost too many examples of that to give.
Kudos though for making the overarching sci-fi plot so integral and the ending as wild, if not wilder (although lacking the surprise) of Asassin's Creed II. Genuinely loved it all falling into place with some crazy plot points recontextualising the whole game.