I look forward for a more robust and durable hardware design. Maybe a ruggerized version?
They already are water proof and can be run over by a car. So unless you want to take a hammer to it, it should be rugged enough.
The first batch of the 4Cs were notoriously fragile. The plastic would break within a few months of normal use.
USB C has a connector that cannot just be the PCB, and so it needs a housing and such.
The Yubikey 4C nano is hardier than the original Yubikey 4Cs were. I've had mine in for months, and it hasn't broken.
That said, I should note that my laptop (Macbook Pro) also decided to provide insane amounts of power to the USB-C ports on the left side, which fried the Yubikey - literally, there was smoke - and melted the plastic. (That's definitely Apple's fault, though, not Yubikey's fault).
So I would use at least 2, but hopefully websites will allow this, and that will probably be the largest bottleneck in the future. I couldn't care less about "SMS backups" or such nonsense, as that completely defeats the point of using a hardware token. Your account will only be as secure as your phone number is.
Many sites allow this explicitly, and will let you view details about the last time each key was used to log in.
Some sites that use TOTP only allow for one "authenticator" to be configured. In those cases, I scan the same QR code into each key.
This process requires you to retrieve your backup key from whatever safe place you store it in when configuring 2FA on new accounts, but that feels like a reasonable trade-off; I don't make new accounts very often, and when I do I can wait to configure 2FA until I have access to my backup key.