And this is where I think they were really smart! Because legendaries have really distinct and unique outputs, they tend to overperform when underleveled. When you're underleveled with a common card, it can underperform because it loses stat checks it should win, but legendaries are more robust.

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.