That's not entirely true. I built a FlatBuffers implementation for Erlang as NIFs so that I could still take advantage of FlatBuffers memory efficiency feature

That's not entirely true. I built a FlatBuffers implementation for Erlang as NIFs so that I could still take advantage of FlatBuffers memory efficiency features, which fundamentally required "unsafe" and mutable access to a byte buffer. I just had to be very aware of exactly how the runtime was going to interact with my implementation and account for it. Sadly the place where I worked was not keen on open source posture, so it's locked away for all of time probably. However, the place I now work is a lot more open source friendly and I expect to take another whack at it, but using Rust on the NIF side this time around instead of C.