If it's long enough it's fine. They key property for passphrases is being able to remember them. But diceware phrases where you actually roll dice are stronger,

If it's long enough it's fine. They key property for passphrases is being able to remember them. But diceware phrases where you actually roll dice are stronger, and has evident verifiability. RNG on computers is still hard, and a proper HSM has a carefully designed source of randomness and mixing. I'd trust Yubikey RNG mixing into Linux urandom much more than a random hardware wallet developer.