For example, let's look at wizard (rare) vs. electric wizard (legendary). Wizard shoots AoE fireballs that do pretty crazy DPS. E Wiz deploys with a small pbAoE stun with marginal damage, then shoots 2 bolts of lighting at nearby targets, doing light damage and stun.

Because all of Wizard's power is tied up in base damage stats, he's a playable unit when at equal or higher level than your opponent, but kinda sucks when underleveled. He also dies to a fireball if he's underleveled, but lives with 1 pixel if equal, making him much more vulnerable if underleveled.