Buck2 is faster than Buck. If you've got nothing to do, Buck2 is significantly faster. If you've got lots to do, Buck2 will start doing it faster and be much closer to the critical path.
Buck2 rules are hermetic by default. Missing dependencies are errors. These restrictions apply to both the user-written
BUCK files and the language rules. Buck2 gives the right result more reliably.
All rules are written in Starlark, with nothing in the core of Buck2 knowing anything about languages. That means that Buck2 users can define their own rules as first-class citizens.