Smart contract managing a device would only interact with devices if their hashes are matching the manufacturer's hash. This would be the first security test. 

The device would then only be able to relay data or do anything if the smart contract gives it the permission to. Which it will only give permission if the hashes are matching.

At this stage you can now rely on the device. 

The natural advantages of the smart contract is the ability to know what it will do before it will do something. This level of transparency gives reassurance to the person interacting with the device.