Stateless so the device itself is "immune" to physical compromise.
Commodity hardware dramatically reduces the chance of supply chain attack.
Commodity hardware also reduces the chances of being identified as a target for having enough bitcoin to come after.
Seedsigner is not immune to physical compromise. By default it has no tamper evident properties. If you're depending on it as your sole hardware signing device, I would strongly encourage locking it up when not in use and using a tamper evident tape. Odds are low that you'll ever experience an issue, but no reason not to add extra low cost layers of defense.