Extremely hard difficult challenge: Avoid bringing up outcasts.
In one case, there's also a question of ECL. How is ECL calculated. Determined, and factored? Stat distribution? Perks? Benefits?
One major glancing factor is Orog-Gnoll comparison. Gnoll is almost far superior due to having two feats, innate ability, yet is classified as +1 ECL while Orog have just one feat, +1 AC, and Fire/Cold 10% immunity ontop but is a +2 ECL because they have 2 extra stat over the negative.
How does this factor, I don't know. But there's one glaring. If we were to compare these races with surface. The ECL calculation is stretched.
Wild Dwarf is 0 ECL, have almost the same statblock as Shield Dwarf- with exception that they get increased saves against poisons, diseases, Use poison feats, 15% fire immunity, and doesn't leave footprints and is capable of -track. (Edited)
Or how a Wild Elf get ALL of the Moon Elf, plus TWO feats (Trackless steps, and Toughness) for FREE at NO cost to ECL. ALSO A +1 AC ON TOP.
Case-by-case. Underdark need a big buff. And also separate Kobold/goblin for god's sake. They're literally almost the same thing.
It's about time we stop caring about the 'MUH UMD' problems, and start thinking about the glaring issues.
So What is ECL? What exactly is ECL factored? It's clear it doesn't even have an actual focus. All races are weirdly judged- and are given weird ups- and downs.
TL:DR - Surface races have more advantages compared to Underdark.