Once you're in offline mode, you don't get cloud sync (nbd, honestly, except for when I get a new machine) and everything else works fine.
So synapse is online by default and has bad error detection (should just fall back to offline on timeout), but doesn't, strictly speaking, require cloud services to work. Not great design choices here, but not as bad as it sounds.
source: I am an owner of a razer mouse, couldn't configure it in windows on S3 day until I changed my config.
So I'd personally be happy if there were synapse drivers for linux, as I do work in linux and I'd love to be able to program macros and whatnot onto those buttons like I do when I'm booted into windows. It is a fairly handy piece of software, and I challenge you to find a better seventeen button mouse.
As for my mouse, I had to use some Corsair junk to configure the button mappings, sensitivity, and turn down their ridiculous LED, but then it saved the settings to the mouse firmware so I can uninstall it and use it just fine on Linux. No more touching it again. Still not happy about it. Would rather it had been some tiny DIP switches underneath.
I have a Corsair mouse, and their software is spectacularly awful. (Their other software is also terrible; like configuring fan speed profiles for the water cooler.) No account required, but the button configuration is horrendously complex and in the end completely useless. I never figured out how to make a mouse button show up to Windows as an extra mouse button. Switched to a DeathAdder, and while their software is also terrible, at least the extra mouse buttons show up to the OS as mouse buttons. (Razer's software is terrible in a different way; the UI is overdesigned and I don't want to type in a username and secure password to change my fucking mouse settings... but once you're in there, the software does actually work well, letting you configure what you want in a relatively straightforward manner.)
All in all, this stuff is all super gimmicky. I want the marketing people to know I bought their product _in spite_ of the software they spent hundreds of thousands of dollars developing. I would be happier without it. The company would be more profitable without it. The mouse is for pointing at enemies and clicking when their head is under the crosshairs. I don't need a "brand experience". I don't need fancy colors or "game optimization". Just translate my hand motion to input events. I'll do the rest. Thanks.