I think it's more of the learning curve.
Morde only has damage and essentially increasing his health bar in combat. You really have to understand not only the damage you do, but the shields and healing that you also get as a result when it comes to fighting. Morde's W can easily turn a losing fight into a winning fight for him because people really underestimate the power of his heal.
And Aurelion? Jesus christ he can do amazing things. His W is not bad when it comes to raw damage, and it can hit enemies repeatedly. He can roam quite well, and his Q can lead to some massive problems for those who aren't prepared. Problem is, if you don't know what you're doing with him, you're just going to get an enemy who dashes into melee ranged and you end up feeling really powerless.
I'm not much of a high rank player either, but my mindset has always been "just because they're not played does not mean they're bad"