I can see firmware updates being a problem.

If the malware can hijack the firmware update process, then it copy itself onto the Trezor (making the device worse than useless.)

I assume it will display a fingerprint of the new firmware and the user will need to press the button (like with a BTC transaction.) Still, a few users will probably be caught out by this.

Unless they Tivoize it (then the "open source" claim is rather dubious, but I think this may be the best solution anyway.)

So don't buy a used one.