Isn't Null's drain significantly weaker (and/or different) than Rogue's?
It is different, Rouge isn't just draining energy, thats a side-effect she can't control, she is copying their biological blueprint and using it to shapeshift; she takes genetic traits, personality traits, and even skills from people she touches.
Null is a sub-atomic energy siphon, his powers are more similar to Shaw than Rouge... to the point that it wouldn't surprise me if they were related.
Not sure I would call it weaker though, Rouges drain is contact based while Null's is an AOE attack. Also Null's drain works on inanimate objects, it can weaken steel and wood, Rouges drain doesn't alter material properties.
It's not a like to like comparison.
If I had to guess, I would say that Null develops powers based on how he percieves them to work rather than how they actually are.
In the official lore mutations are a form of hyper evolution that gets triggered in response to extreme stress and/or radiation. There have been many X-men storylines where the government produces mutants by torturing people in unique ways; successful x-gene activation is usually less than 1 in 1000 so it's not a very cost efficient method given that 99.9% of the subjects don't survive and the population with viable x-genes is already small.
Under normal conditions it's one mutation per lifespan and then the x-gene resets for the descendant while some of the abilities of the parents carry over. Cable and Kurt are good examples of this.
However, some mutants can keep their X-gene active for over a decade, they don't get new powers but their current ones evolve. Emma is the most famous example, she has two separate mutations and in some continuities develops TK as an outgrowth of her telepathy.
Null is probably like Emma where he has a power set that hasn't fully formed and is still adapting to trauma, the similarities to the girls powers is more likely correlation than causation where he was put in specific circumstance where powers like theirs were needed to survive and developed a similar response. It's called convergent evolution, see hummingbird moths as an example:
