Not fatal, but very annoying. I have a Netgear "Nighthawk" USB WiFi adapter plugged into it, and while it supports Mac, and by extension Linux, it's cranky to get set up. I should have plugged an Ethernet cable directly into the router and ran that to this PC, as I think it may have borked the drivers for the WiFi adapter last night, losing connectivity.
Running things on Good Old Ethernet for now until I get the driver issues sorted out. I like the new menu layouts, but as with all "upgrades" a few things got moved (simplified, really) so it'll take me a few days to get used to it.
And it definitely borked all the user info in my Ham Radio programs, so now I have to go back and set them up again for my station, Again, annoying but not fatal, and I now have all the newest versions of the software I run.