The Secure Boot feature (https://github.com/f-secure-foundry/usbarmory/wiki/Secure-bo...) in concert with full disk encryption prevent anyone from extracting keys.

When used in combination with user supplied passphrases this leaves little chance for physical attacks.

The two external security elements have active protection against glitching and side channel.

> Is it secure against key extraction from somebody with physical access to the hardware?

it is supposed to be.

Seems unlikely. Pretty much everything is vulnerable to voltage glitching unless specifically designed to mitigate against it.