I am contributing to a project by a SpacetimeDB community member to get Rapier running within Unity. Instead of trying to get Unity PhyX running on the server, we just use a deterministic Rust physics engine on both client and server. It is a mostly complete drop in replacement as it stands and is deterministic (Unity's PhyX is not). We'll keep improving it!

https://github.com/Daxode/rapier4unity

That's great, especially the deterministic part. Did you run into determinism problems with two different CPU architectures for server and client?

We rely on the guarantees of Rapier's determinism. I haven't done extensive testing on the determinism claim yet, but it has held up so far. https://rapier.rs/