In comparison to the pay version are at least this restrictions:
Includes Generation studio wide management software for assigning tasks, tracking assets, and versioning shots.
Includes Avid Connect plug-in for using Fusion from the Media Composer timeline on Windows.
Resolution independent with support for images over 32K x 32K
-Yes. Limited to Ultra HD Output
And I think I heard that for an unlimited use of nodes you need to buy it. But not 100% sure with that.
at the low cost of $995. Fusion Studio customers get unlimited free render nodes and other multi user features.
I tried it now for the first time ( on a really really crappy PC), and it seems pretty fast. It makes good use of the Ram and the cores.