Can I use it as a Yubikey alternative?
I can confirm that Trezor Safe 3 can be used for U2F authentication, it will support FIDO2 standard. It will work very similarly to the Trezor Model T. The specific articles for Trezor Safe 3 are being prepared right now and will be released before the new product is delivered to customers. For now, you can check these articles for U2F with Trezor Model T:
Frankly speaking that’s the main reason i am Grabbing one myself.
Technically speaking at least webauthn isn’t the side the trezor even interacts with (as webauthn is just website - computer part), the computer - device part is u2f or fido2.
The trezor One supports sadly only u2f (I’d love to see that change someday), so you do not get the modern features like user verification or resident credentials,
but both the model T and Safe 3 claim to have fido2, which is awesome. To what extent in terms of subfeatures, i will definitely try myself on the ts3, frankly speaking i cannot wait.