> it only has a text encoding as long as you can guarantee you don't have any unicode?

The format is technically a binary format in that length prefixes are counts of bytes. But in practice it is a textual format since you can almost always copy-paste RX values from logs to chat messages to web forms without breaking it.

unciode doesn't break anything since strings are encoded as raw unicode with utf-8 byte length prefixes. It supports unicode perfectly.

If your data only contains 7-bit ASCII strings, the entire encoding is ASCII. If your data contains unicode, RX won't escape it, so the final encoding will contain unicode as UTF-8.