At some point you have to roll off the brakes and back onto the throttle.
At this point, if you are on the brakes and the clever thing rev matches for you, you are only on the brakes and the throttle is constantly dropping as you are not applying it. With proper heel toe at this point, you seamlessly go from brakes to throttle to not upset the balance of the car (the whole point of heel toe I might add). But with this system, you are on the brakes and off the throttle, which you now have to "get back onto". I can see some people being far too blunt with this, jabbing the brakes, shifting down and whacking the throttle back on again. Completely upsetting the car balance and stacking it into a kerb/tree/ditch.
I'd have thought the TCS would kick in at that point. And they'd disappear off the road by not heal and toeing anyway in that instance. I think it gives a good opportunity to shift the left foot across to the brake earlier if not playing heal and toe, especially when you're not Ari Vatenen in the footwell!
Left foot braking eh, hmmm, yeah you could try that. But I'm going through the window with DJsickboy in that case!
I guess the TC will save them at that point, you're right.