Both are smartcards. Yubikeys also happen to support the USB HID CTAP protocol, which is something unprivileged applications can usually talk to directly on most operating systems. Most other CTAP transports are just smartcard transports (APDUs over PC/SC or NFC). Yubikeys also have a PIV applet, which isn't used for your typical enterprise X.509 PKI, and an OpenPGP applet, which typically isn't used.

PIV, GnuPG, or PKCS#15 are probably better comparisons. SIM cards use symmetric cryptography, which means you can only authenticate yourself to a single entity.