The engineering issue is a lack of accidental touch sensing. Mac did this with the caps lock key (which is arguably easier to implement). The feature feels half baked.
I'd argue that the Touch Bar implementation is fully baked but utterly useless.
There's a ton of engineering and polish that went into that feature — from the way it can securely render the price of a purchase through Apple Pay to the way it communicates and transfers its frame buffer, to the UI components it can smoothly display.
