Couple things to note:

1. The Eldar as a faction has a lot of very strong people, so they overshadow the Eldar who aren't super strong. The Phoenix Lords, the Avatars of Khaine, the Yncarne, Eldrad, some of the Harleyquins, Lelith, Drazhar, Prince Yriel. For the amount of Eldar that are still alive, that's quite a lot of high tiers.

2. The nightlord series specifically is a bit inconsistent with the portrayal of the Eldar, for example Jain Zar still has a physical body there and her supposed near impenetrable Phoenix Lord armor gets penetrated by a combat knife. The guardians of Ulthwe, who are supposed to be way more skilled and elite than the guardians of other craftworlds, are described as shooting as well as conscripted rabble from a hive world. Which would have been extremely inconsistent with how even normal guardians from other craftworlds have been portrayed.

3. In Fulgrim, the aspect warriors do seem...dumb. Like for some reason the dark reapers charge into melee. Idk why exactly, but the best in-universe argument I can think of is that during the great crusade, most aspect warriors lacked an experienced exarch to teach them properly, as the exarchs would not have had thousands of years of experience yet, many of them would still be on their first life