[jwr]

IMHO the YubiKey is not useful for any of those. It's excellent for storing OpenPGP keys and U2F, reasonably good for X.509 (as much as expected for X.509 I guess), and not good for much else. Using it for TOTP IMHO makes no sense, it's better to use your phone.

[jfim]

Using it for TOTP makes sense if you have more than one phone or want to use TOTP on your desktop through Yubico authenticator.

[jdeibele]

Authy is excellent for this. I've got it on my phone and tablet. I'm reluctant to use it on my desktop because I don't want to type in a huge password but I regard my 2015 MacBook as less secure than my devices that are protected by touch. You might be OK with that or have a laptop with touch ID.

[Boulth]

Exactly. Additionally phones can be rooted and that exposes the underlying secret but Yubikeys are tamper resistant.