Honestly, Nnoitra and Nel are a place where Bleach canon and Number None canon don't line up particularly seamlessly.
In base Bleach, Nel was the Third Espada at some undefined point in the past, at the same time Nnoitra was the Eighth Espada. Nnoitra was being sent around by Aizen to scout outlying hollow gatherings for really strong recruits and/or vasto lordes, and Nel was more or less hanging around and blocking him whenever he tried to commit a massacre for fun, easily beating him down. Then, with Szayel's help, he managed to cripple her, after which point he clawed his way up the ranks to become Espada 5. When Nel showed back up and gained her adult form, he claimed that the power scale had changed, and ranks that existed when Nel was an Espada were not comparable to the current ranks. Despite this fact, and despite Nnoitra actually having gotten obviously and non-trivially stronger in the interim, adult Nel pretty decisively thrashed Nnoitra in their fight; he was always on the defensive, taking nasty hits and unable to hurt her back, and the only reason she didn't kill him was because she ran out of gas and reverted to being a helpless child. This, even though all the time Nnoitra was training up for their rematch and growing stronger, Nel was wandering the desert in a mentally and physically crippled state not fighting.
In Number None, we don't know much about Nel, but Nnoitra, long before becoming an arrancar, was a roving disaster, who butchered his way straight through a literal army of hollows including a dozen or more gillians and probably half a dozen adjuchas. He was the Killing Moon, widely regarded as absolutely invincible and murderous, recognizable from a long ways off. The strongest adjuchas to ever live. It's really hard to reconcile that with Nnoitra who was, after being made an arrancar as one of Aizen's special wishstone freaks, only the eighth most powerful Espada, in a group of Espada overall less powerful than the current batch. Plus, a group of Espada who don't really believe in growing more powerful via training, even though Nnoitra clawed his way up the ranks and got considerably stronger in the process.
If I was going to try to reconcile it, I think I'd have to go with something like saying Nnoitra's demasking seemingly went poorly, and he actually lost a lot of power when he became arrancar, but he gained it back steadily. Thus, Rank 8 Espada Nnoitra is essentially a period when he was crippled by being an early Hogyoku experiment, and Rank 5 Espada Nnoitra is more like his power level was as the pre-arrancar Killing Moon Nnoitra. Nel was just always badass, but not particularly murderous; she'd rank somewhere above Nnoitra but below Ulquiorra in the current time period Espada. She wasn't legendary as the strongest Adjuchas because she didn't go around killing armies for fun, but she could have if she wanted to.
