User blog comment:B214/Question and Answers 6/@comment-185.112.105.215-20161218170008/@comment-9595671-20161220171955

Yeah, being able to handle certain jobs and roles, as well as maturity and wisdom play a role. When their training in Hell began, I think Azazel helped show a good example that Rias may be somewhat mature, she's still a teenager.

That's probably why in most DxD fanfics where Issei is much stronger than her, I see her being shown as inexperienced and not very wise in how to train herself and the rest of the ORC.

A common example I see is she's wasting too much of her power fo Destruction in one spread out blast which may actually be diluting it's power, rather than focusing it and using less power in the process. This is likely the same basis behind why Rias is not qualified to be at Ultimate-Class.