Solid 2FA I have been using the YubiKey for over 2 years now and it has saved me and my company from potential damage on multiple occasions. Just make sure to never lose your key ;)

Using Yubikey introduces the risk of the rightful owner of account is prevented access. It is a problem that Yubikeys cannot be duplicated for backup. They are so secure that even the owner of an account can be locked out, lol. That's not good. In case of lost key or hardware malfunction there is no way to access your account. Unfortunatly two different keys often cannot be registered for the same access. Using Yubikey introduces the risk of the rightful owner of account is prevented access.

Yubico's prompt refund I have a higher import duty issue with UPS carrier for an Yubico order and requested them to cancel after 5/6 days. They cancelled and refunded promptly.

series 5 nfc buggy bought 2 series 5 nfc totally buggy to a point where yubikey-manager couldn’t read the seriol# of both keys anymore and AOL rejected both keys in that condition. support totally ignored my issues and closed my tickets before solving them How does anyone trust a company or a piece of hardware that buggy??

Their product didn't work Their product didn't work. Customer service offered a partial refund (excluding shipping costs). That's unheard of. When a product is returned, all payments are refunded and some will even send the client a free shipping label. When a product is faulty, that's goes without saying.