To confuse matters further, many FIDO keys can function in both U2F and FIDO2 mode with each method producing a different type of seed on the server. That said the registration process itself is still fairly straightforward, and there is currently an uptake in interest in FIDO keys (google, microsoft, etc). Personally I believe the longer term issue is not with the technology itself, or even the documentation, but using a USB based authentication device (and with the security concerns that USB devices bring with them).