Here's a tip if you play a lot in single player HU you should temporarily empty the folder save/game. i was shocked how data garbage has accumulated here. This comes from the automatic Quicksave every two minutes.
I wasn't as impressed with Onward as I'd hoped. Don't get me wrong, it's a fun game, but I prefer Zero Caliber. This is probably due to the fact I can only play in 10-15 min bursts, and ZC is much more suited for short play sessions.
I think the game's 'servers' in the more colloquial sense are sperate to the game validation servers which is part of the Uplay service, I believe. So provided Uplay is still connected, the game will still be playable, albeit in single player.
Quick way of testing I guess is to block internet access from the game .exe in windows firewall but keep Uplay open and connected, if the game runs, then it will be fine when the gameplay servers go offline.