Re: Safe enough - IF no third party code
If there is a "secret" engineering backdoor then this is a much significnat problem than spectre or meltdown.
If a device is not intended to allow execution of any software other than the device software then it is a catastrophic security failure if a means to do so is found. Once an attacker has the ability to run arbitrary code on a device not intended to do so then there are almost certainly much eaiser and more direct ways of accessing information than spectre or meltdown.
It makes a lot of sense that appliance manufacturers take this approach. They may do a good or bad job at it but that is a different question.