First off, commander generals for the most part are not banned for being broken. General like
Meren of Clan Nel Toth and
Zur, the Enchanter are
much closer to "broken" than pretty much every banned general aside from Leovold, and it's highly unlikely that either of them will ever be banned. And secondly, I'm going to respond to you as though we're talking multiplayer games, as the banlist as ran by the RC is not designed for nor all that relevant to 1v1 play.
But going on to Najeela, she is certainly not overpowered. Non-combo aggro strategies are pretty terrible in multiplayer as a whole, and especially in Commander as the life totals are doubled. So, the only way that she can have any chance of winning against other high-powered decks is if she goes the combo route. And some facts about the combo route:
1. She has 8 cards
that can combo off with her, although every single one of them requires multi-turn setup.
2. With a god hand, she can theoretically win on T2. In most games, she can establish a potentially winning position by turn 4-5. Compare this to other fast combo decks like Hermit Druid which can win T1 but usually aims for T2-3, Protean Hulk which is in the same situation but can potentially win T0, and Narset which pretty consistently can win T3-4.
3. Common ways to stop her: well-timed spot removal/counterspell on either her or her combo cards, mana denial, getting your general out early to block her/her tokens, wraths, and global creature hosers like
Humility or
Ensnaring Bridge. Many of the above things are exceedingly common in Commander as a whole, and all of them are especially common as the overall power level of the table increases.
4. The vast majority of powerful commander generals have one thing in common: card advantage. Most good generals are either CA engines or powerful removal/lockdown pieces. Najeela provides zero of either of those qualities, making her far less resilient than other powerful generals. She also has no way to protect herself, and whatever impact she had on the board can be instantly eliminated with a wrath.
Ultimately she's in that weird limbo spot of being too slow to be a glass cannon combo deck, but not resilient enough to be a more midrange/control combo build either. She's also hurt by the fact that she can't dodge removal, and outside of significant help from other cards really doesn't do anything except affect directly life totals.
In a casual meta or one that is otherwise unprepared for her strategy/tactics, she certainly has the power to steamroll the table. However, the cards/strategies that tend to neutralize her are things that most competitive decks are already doing a lot of, so she's at best the dark horse in those games.