She was overconfident and arrogant, and got herself compromised, forcing Ainz to kill her in order to get the debuff off her.
In the books you get her perspective, and her confidence gets completely destroyed by the mind control incident. She eventually develops a pretty healthy level of caution similar to the one Ainz exercises.
Yeah, she wasn't one, she was incompetent and fucked up.
The key word there is reliable.
Lupusregina made a similar mistake, she fucked around and found out, and Ainz became furious with her because she didn't take things seriously when told to keep the village safe.
It was a human village so she thought it wouldn't matter if some of them died, so she didn't try very hard to stop it from happening. So even though she had the abiltiy, she still fucked it up.
It didn't matter that they were maxed out in terms of strength, when their heads weren't in the right place.