Nitrox is 100% fatal at depths beyond prescribed limit. The whole point of Nitrox is to increases the oxygen content in the gas to minimize nitrogen absorption. A side effect of this is that you will get to hyperoxic poisoning thresholds earlier on a deeper dive profile. This is why Nitrox requires additional certifications, to help understand these limitations and learn how to calculate your max operating depth. (when using nitrox wihtin these thresholds I find I have more energy at the end of my dive).

To go deeper you need to blend in inert gasses and decrease o2 and n2 to keep partial pressures below fatal doses. This means most commonly mixing in Helium to cut down on over o2 and n2 in your gas mix. Human tissue I guess doesn't absorb helium the same way it does n2.

Source: Open Water Scuba Instructor with multiple agencies... I havent taught in about 5 years though so some of this information may have changed, especially in light that global helium shortages (or US stock piles?) may have driven the price of He up to high to be used for mixed gas diving.