My ranking of Morning Musume members in terms of standing out:
1) everyone else - did a pretty good job at leaving their mark on Morning Musume/H!P in singing, dancing, acting/voice acting, appearing on variety shows, modeling/image, or presenting a memorable personality.
2) Kamei Eri
Now, I am not going to say that Eri stunk at anything, because that would be wrong. However, she had the misfortune to enter Morning Musume at a time that anything she could have stood out at she was going to be ridiculously overshadowed.
For example:
1) Singing - Gen 6 of Morning Musume not only had Reina Tanaka but also had Miki Fujimoto (remember her?). No way she could have stood out with those two around.
2) Image - She was left in the dust by Miki, Sayumi, and even by Reina (who got off to a slow start).
3) Dancing - She tried. They even tried to give her a dance solo during TOP! That did not work out so well.
4) Personality - Again, misfortune. If Reina hadn't been there, she might be remembered as a memorable tsundere girl. And, a memorable aho girl? The group at the time she joined was full of aho's.
5) Voice acting - Misfortune times two. Not only does she have to be compared with Reina (Onigai My Melody Karara, Kaitou Reinya) and Koharu (Kirarin Revolution), but the toy sales for her anime (Jewelpet) were so low that it almost bankrupted the toy company.
6) Variety shows - Okay, she kind of stunk at this.
So, my conclusion is that even though Eri had a really long run in Morning Musume, she didn't really stand out.
(On the other hand, an idol's job is to make people like them. Nothing else really matters. So, if you like Eri, then she at least did one thing right.)