I think you just answered your own question. The fact that Nebula sounds so very different from 'regular' plugins, is that Nebula is 'sample'-based, meaning it relies on a stream of impulses, instead of code. Also, harmonic distortion from the original unit is also being sampled, which gives depth and it 'grips' the sound. Plugins like Waves S*L/UAD, etc. are actually a bunch of digital coded curves, and have nothing to do with the sound of the original hardware. Try boosting 10dB at 10KHz with any 'coded' plugin, and the sound, as well as your eardrums fall apart. You can actually do this with Nebula, just like the hardware!
I could go on like this for hours, but you'll get the picture!
The difference between heaven and hell ...I have waves v3/v4, very plastic . has no weight or depth. stoned. nebula is deep, fat in great detail. Not 100% hardware, most certainly 95%. The others not so. I love nebula, i will die beside him. Um beijo!!!!!