Which units would you replace in the Dark Elf faction?

I would replace the Scorpion Riders with something else. In my opinion, Riders are the most useless unit – they are large in size but have virtually no defense.

Please forgive me, but I completely disagree.

Their large size increases the number of targets they can attack (6 out of 10 cells – that’s significant).
When attacking after moving a long distance (5 or more cells), they COMPLETELY ignore the target's defense. Combined with high attack and damage stats (which is amazing for a level 4 unit), and the fact that heroes usually have good attack stats, they are very effective. They also have very high speed and initiative.
The Lizard Bite is very effective, especially when the hero is Sörgal. (With eight Riders and five Furies under Sörgal, I can easily handle a crowd of any monsters up to level 6, except for Pelors).

And they are incredibly effective in the final battle.

In my opinion, they are the most powerful level 4 unit in the game when you have 20 of them.

Maybe you just haven't figured out how to use them effectively;
Or maybe you simply don't like them for aesthetic reasons.

As for their low defense, if you increase their defense, it would be an overpowered unit.