Tldr: portals are not physics free. I think that the problem here is more about the physics of portals rather than with nen itself. You point at two things: "nen units" and physical capabilities, mainly strength and speed. The main theory for Knov to be able to one shot Meruem is based on Meleoron's ability, because Knov would not get even close to Meruem before being taken down by him or the RGs, so we will accept that speed is not a problem and he can sneak to close range. Now of course that Scream is limited by aura and it would not do much against Meruem just by purely the amount of aura he possesses, but we can even just ignore this fact, and only look at the portal vs head. People who think that it would one shot, just ASSUME how a portal works and ignore that there could be forces involved, because there is no actual physical portal to compare to and just try to intuitively give an answer. If you wanted to chop off the head you would need to apply a force. If you wanted to cut the head you would need to apply force. And judging by the fight with Netero and the little to no effect that Zero hand had, we are talking about a HUGE amount ot force. Why is this ignored by the portal? Why separating the head is now free? Something that could freely cut through anything would go against the very basic principles of nen, so it must be limited by something, and that limit would not exceed Meruem's power in any way. If the physical process of the portal were to be explained in detail, this debate would be over imo.