I have used Yubikey with Nextcloud, via OTP and via U2F. Works quite good. It’s nice to have different modes and also use certificates. If it is just for U2F, there are a few other and cheaper devices.
I would appreciate if anyone can comment on which 2FA-related alphabet-soup anacronyms Nextcloud 14 supports (eg. FIDO U2F/CTAP1, WebAuthn, etc), when using some kind of Yubico-like dongle, (which might also have NFC, for use on mobile devices).
Please dispel the perplexity on what will and will not work, at present!