From a maths point of view it is very sensitive to the inputs making it rather unstable. On a dive a small change in depth can make a large change in NDL. Around the on/off gas limit depth it is tending towards a division by zero and so tangental. Using it as measure of risk “leave 5 minutes of NDL” is not really that helpful, that is half your nitrogen at 40m but only 10% at 20m.It is a perfectly good indicator so long as you don't try and redfine NDL to mean something it is not.
Yes, it is for a square profile. Like tables.
Yes, it assumes a particualar descent and ascent speed. Like tables, even computer-generated tables where you have to specify your descent and ascent speeds.