this is not how USB-C works. you might end up with a cable that doesn't support high voltage, but it simply won't upgrade to a higher voltage and your device will charge slower or not at all.

I agree that it's frustrating that all USB-C cables aren't interchangeable, but to imply they're potentially unsafe is wrong. There was a bunch of out-of-spec cables on the market shortly after USB-C came into use with the nexus phones and the macbook pro, but that problem has almost entirely been solved, and is still a much smaller problem than cheap aftermarket USB-A charging bricks sold by the same sketchy vendors.