I played the original to death. But my playgroup noticed that the game definitely had balance issues. Zerg rush Scourge is broken for example. or Protoss should tech to Archons first. Does the expansion help to fix balance issues?

Archons have been nerfed slightly, they cost an additional gas and only have a support value of 1.

The scourge sacrifice mechanic has been overhauled and now works much differently. They also cost one more gas.

New balance issues have arisen however. Some leadership cards have power issues at different player numbers.

Over all I feel broodwar helped the balance of the game, but without constant revision every game stays somewhat imbalanced. And that is a good thing, I feel like starcraft does very well at allowing players to do powerful and impresive things while still maintaining a balance betweent the factions and races.

As most of the new features are modulare you are the one to decide what the game exactly make it a perfect balanced game. All in all brood war is more balanced than the basic game, also the players have a lot more ways to be victorious. The most unbalanced thing in brood war in our opinion are the leadership cards, cause the players get insane advantages in stage 3 of the game, so it has no more to do with good gameplay of the players, but a bunch of rule variants included in the rules may change the game experience to the joy of every player.