I don't understand how you think it's different. Omaeda and Nano would both be people with talent in only one shinigami art.
So does Hacchi, but it didn't stop him from becoming a powerhouse Kidô practictioner. To a lesser extent, the same can be said of Hinamori, who was acknowledged as a master at Kidô by Kira and Hitsugaya.
In Nanao and Omaeda's case, you comparing one who has talent to someone who doesn't. Omaeda has had tutelage under Sui-Féng, yet by his own admission, shunpo is the only thing he's good at (i.e. running away). That's why he said he wished he was multi-talented like Shuuhei and the other lieutenants.
Nanao has a gift at Kidô, which Shunsui and Lisa both recognized, which is why they gave her books on the subject. Chances are, she might've been better off in the Kidô Corp. where she could've learned more advanced spells (and combat tactics) than what was in those books.
She has a suitable attitude, but doesn't have a good sense of how to apply her skills towards attacking an enemy.
Because she lacks training and experience.
Imagine what might've happened had Tousen held Hisagi back, the way Shunsui did to Nanao. He'd still be that scared kid that Kensei found, and he'd still panic like he did during that field exercise where he, Renji, Izuru, and Hinamori were the only survivors. But Tousen helped him overcome his fear and under his guidance, Hisagi became one of the most capable lieutenants in the Gotei.
Instead, Shunsui stunted Nanao's growth by preventing her transfer, then coddled her by not allowing her to fight. Which is why I called it f*cked up.
You could try to argue she would get better with experience, but if we look at the nakama, that doesn't seem to happen.
I can easily counter with Hisagi, Rukia, and Sui-Féng, who were all in Nanao's position once. They were allowed to train and hone their skills under proper tutelage. Look what they each became. Especially in Rukia's case, because once Byakuya stopped holding her back