I have it working on the few W8.1 machines that we have, including a Surface Pro 3. The only hiccup that I had was when trying to install it over wireless and the installer not fully completing, even though it registered on the console as being installed and online. The client had the services, but no other traces of Avast being on the machine (nothing in the task bar, start screen, etc.).
It (annoyingly) would leave some files in the TEMP folder on the server and client, which when I tried to re-deploy the software would immediately make it fail. Deleting the files out of the client machine and server (which required me to stop the enterprise services) seemed to clear it up.