The biggest problem probably isn't modifying the transaction, which is pretty easy to catch, but using predictable keys in the hardware or somehow leaking bits

The biggest problem probably isn't modifying the transaction, which is pretty easy to catch, but using predictable keys in the hardware or somehow leaking bits of the key in the transaction. This way they could watch all the value stored in their wallets and steal whatever they wanted by making it look like you did it yourself when one had a large enough balance to be useful.