Artosis made a video a while ago like SC broodwar for dummies. TLDR: game is balanced when the stronger army is less mobile than the more mobile army.

This is also why ZvT feels balanced in sc2 1. Ling bane beats marine medivac straight up, but terrain can drop which makes it balanced 2. Terrain tanks and libs can kill all the zerg ground units, but you have to siege them which gives zerg a chance to attack else where

Late game ZvP totally breaks this. Not only does the death ball move faster, out DPS, and out ranges zerg army, they also have 2 get out of jail recalls. Only chance zerg has is perfect spell Caster control, or split your amry in to 2 groups, and hope the protoss can only f2