This is exactly how I describe them to many as well. I just wish there was a USB key with the durability of a metal key.

Somewhat personal ancedote, but I've had pretty good experiences with Yubikeys. I've had one on my daily keychain for over a decade now. Its been run through clothes washing machines too many times to count, I've left it out in rainstorms, its been baked in the sun on hot summer days multiple times, its been dropped in pools, its been run over by a car, I've dropped it some pretty significant heights while hiking. It is still working just fine. That and a PNY 16GB metal keychain flash drive have taken a ton of abuse and are still working just fine. Other than a higher melting point or ability to take high energy RF I don't imagine there is much more abuse a traditional metal key would have compared to what I've done to my Yubikey.

Maybe it’s the old USB version? I haven’t had so much luck… I had a couple of nanos, and those broke pretty quick. More recently, I switched to a usb-c version…. And while it still works and stays attached to the key ring. The plastic housing has broken already. I got a corp branded one now, that I’m trying and seems a touch more robust

I've only had the full USB-A style ones. Every device I've had that I've needed a yubikey with has either a USB-A or supported NFC. The >1 decade device is a Yubikey NEO, I recently got a Yubikey 5 USB-A variant and also have one of those FIDO-only blue Security keys in USB-A.

What kind of failures did you have for the nanos? They just became unresponsive? Did they suffer any obvious physical failure or any particular kind of event cause their failure?

If there was a NFC only version of these (and laptops came equipped with NFC), then we could make a block-of-metal key. No holes and I mean the keys with no buttons. Would be water proof and crush resistant. Assuming that you can send NFC signals through metal.

Not quite. It's easy to create a duplicate of your metal key, and many people do exactly that to avoid a situation where they lose it and have no way of opening their door. The fact that you can't do that with USB keys is my biggest gripe with them. I understand that inability to copy even if you have physical access to the original has its security advantages, but 1) most things don't actually need that much security, and 2) it could still be done by e.g. allowing you to buy them in premanufactured batches where all keys are identical.