Knock control is lame to the point its hardly worth having.
As for limitations, it will run 1000cc injectors and idle fine usually, your not limited by Z32 maf either, there are other ones that will work which will read up to 700hp, although I have not personally tried these.
I can do a basic launch control of stock ECU which is pretty good fun, though I don't really push that much as I don't want people blaming me for broken gearboxes...
What nistune does do is create a brill interface to enable reprogramming of the stock ecu. No other solution comes close to this.
As for saying "sorry, I can't do that" I never yet have, £ for £ nothing comes close. If you need maps for different fuels then you use a multiple rom board with a switch. Where it can fall short is that it does not have built in logging facilities, and lack of MAP sensor support, but actually these ecus are great. I frequently put them on totally unrelated Nissan vehicles as they are so good at making an engine run reliably. I have had them running in Micras/Almeras/Primeras, with 1.4/1.6/2/0 engines. Which on the face of it are totally incompatible (especially the Micra/Almera)
It would be nice to have a MAP based gtir ecu. This is possible to, but slightly more awkward. Kind of makes me tempted to look into a way to do this
lol.