I've been running Arch on multiple machines for 10 years at this point. Been through the systemd transition. In my experience there is very little breakage, and it's easier to deal with than a huge upgrade every 6 months or more.
I'm guessing Valve will not deploy Arch packages directly to the Deck, but instead do weekly/monthly releases that are properly QA'd. It should be very smooth while giving them flexibility to deploy the patches they didn't upstream yet.