Balanced Units - A minimal balance mod for units

2. Archers: damage 2-6 (was 2-8)

Against. With a human-sized armor-piercing radius, archers become a good alternative to their counterparts.

Early units are in demand, so can we abandon this change?

Indeed.

The ability change is perceived more easily than stat changes.

Not at all. I don't know what's going on in the minds of so-called "pros," but for casual players, it's usually easier to accept changes to stats.

Swift Riders: 100% chance to hit (was 25%), +agility

I'm all for it. As an alternative, remove the agility bonus but increase the damage to the level of the upgrade.
I still don't see the point in nerfing invisibility and I'm all for buffing higher vampires; otherwise, I have no questions.