I'm afraid we're going to have to agree to disagree.
TA was balanced in a different way - every unit scored based on speed, power, armor, construction time, material required, and so on, therefore, adding a unit - or ten - didn't mess with the balance, as long as the scores matched up. And they did.
I never bothered with the storyline in SC, because after my own units accidentally SHOT EACH OTHER while the enemy ran between them, I was so disgusted that I uninstalled it.
TA is a game founded in excess to force the player to concentrate on strategy instead of micromanagement. The AI takes care of other things for you - for instance, you can take an aircraft factory, make it a group, set a patrol path for it, and all units it produces will be a member of the same group, attack anything that comes in view, return for repairs if damaged and return to the patrol path when repaired.
Starcraft *now* is a finely balanced game. In March of 1998, it was absolutely not. It's had 23 patches and one major expansion pack since then. If they hadn't gotten it right by now, I'd be very surprised. Even now, the fact that they can't change or add units without messing it up totally doesn't mean it's finely balanced, it means it's a huge house of cards - fragile as hell.
I have also played both extensively - as well as every other RTS game to come out since the genre was invented. IMO SC wasn't worth playing until 8 months after it's original release, when they had fixed the balance and patched the bugs and exploits to a reasonable level. Oh, and charged us for another expansion pack to fix the problems that shouldn't have been allowed in a release in the first place.