A small addendum: Jolt, the other new-ish free physics engine this would be competing against, is also deterministic (with some caveats that Box3D hopefully avoids) and more performant than contemporary commercial offerings (the benefit of being new and not held back by accumulated cruft and early design decisions such as obsolete threading models).

What sets Box3D apart is that, like you said, many devs are already familiar with the 2d version of this engine and Box3D mimics its APIs aside from the added dimension. This makes it very easy to pick up for a large portion of the indie crowd.