You compared Dark Riders to Paladins! That's quite a comparison!

You're mistaken; I wasn't comparing them. I was just answering a question.
Regarding

What if you put Dark Riders against any other unit of the same level?

putting them "nose to nose," the most important ability won't be as significant. The stat increase is modest, and the cost isn't the lowest.
Imagine a large battle between Dark Riders and another unit. Would you send the Lizardmen into the thick of it, knowing they're likely to die? I don't think so! In such battles, archers or griffons might be more useful. However, in smaller battles where there are few enemies, I agree that you can experiment.
Always with respect to everyone, THE Lord.