That said, I've been looking a lot into NFC with asymmetric keys since I wanted to use it as a 2nd factor for everything (including phone unlocking, unfortunately that doesn't seem to be happening any time soon).
NFC has a couple implementations: tags (basic, unencrypted, cloneable), symmetric encrypted (all your readers need a shared key) like Desfire... I think these aren't clonable?, and asymmetric which basically requires it to be a full JavaCard NFC chip.
Yubikeys are JavaCards basically. Testing with my computer nfc reader doing a signature is fairly slow (a second or two).
Actual card JavaCards are something like $4 in bulk, and $15-20 for small batches. I'd really like a wrist-band or ring form factor, and I was thinking the dangerousthings implants are small enough they could be stuffed into a band.
You can get a "pocket band" and put a yubikey in it, which works okay. There also are a lot of smart rings, but they're either not passively powered by the NFC connection (need to be charged) or are simple tags and not JavaCard-capable.
That said, they have a bunch of products and an awful website and I couldn't tell what the difference between products was, either (and importantly, which ones were JavaCard and which were basically just tags).