YubiHSM pros

Similar purpose, thus the right threat model
Per key permission flags
No custom OS kernel level driver required (USB CDC)
Well-documented APIs, sample code
Low cost ($500; other HSMs may be $10k to 20k EUR)
You need at least two for redundancy
Independent formal analysis of the Yubikey protocol
"YubiSecure? Formal Security Analysis Results for the Yubikey and YubiHSM" by Robert Künnemann and Graham Steel, INRIA
Assumes "that the implementation is correct with respect to the documentation"
Found an oversight, which Yubico has since released a security advisory on