Is Moto concurrency safe?

No. Moto is not designed for multithreaded access/multiprocessing.

Why am I getting RUST errors when installing Moto?

Moto has a dependency on the pip-module cryptography. As of Cryptography >= 3.4, this module requires Rust as a dependency.
Most OS/platforms will support the installation of Rust, but if you’re getting any errors related to this, see the cryptography documentation for more information: https://cryptography.io/en/latest/installation/#rust