OG NG2 had projectile spam, explosive ninjas were extremely deadly in MN, there were bosses that had adds/minions when at a certain health causing the game to dip once more with blood splattered everywhere in MN, there are times where you'll die from game influx rather than the horde. Was a pretty unforgiving game in all aspects that's even game related.
Free Step Dodge
DestructionBomb
Despite that I hold OGNG2 in high regard, I still have to give some credit to Sigma 2 that it's probably the most stable and functional of the two, but they pretty much sacrificed and took step backwards for the sake of it and the challenge felt lessened/weaker, but it's also subjective for how people want the challenge to be (more enemies or stronger enemies). Then there's NGS2 Vita but I don't talk about that one.
Force_of_Nature
OG NG2 is harder easily for the reasons that DB mentioned. NG2 is horrendously unbalanced on the MN difficulty due to too many enemies at once, projectile spam, and frame rate slowdowns (which also happen in the PSVita's NGS2+). NGS2 on MN felt like a better experience, though admittedly dying by one boss throw is pretty iffy.
One of the reasons is that there were more enemies to handle on screen on top of occasional frame dips here and there that was a pain. NGS2 on the other hand reduced the enemy count because of frame dips, so to balance this they buffed their health and damage. Which is not an entirely bad thing, but fighting 3v1 (Sigma 2) is easier to abuse the system and the AI compared to 6v1 (OGNG2).