I actually finished SC and BW campaigns doing nothing other than zerglings for zergs, zealots for protoss and firebats for terran (marines are too powerful; so I decided to go for firebats).
The entire game was quite easy even with this unit choice handicap. But when I arrived at the last mission of the expansion pack of BW. It was hell. The mission is as follow :
You need to kill 3 overminds; each one with a special ability :
1- Your entire base is surrounded by invulnerable sunken colonies. The only way to reach that overmind is through a very long path of invulnerable sunken colonies.
2- Once ever 2-3 minutes a boss ultralisk would spawn and attack your base. That ultralisk takes only 1/2 HP damage per firebat hit, has 800HP and kills the firebat in a single hit.
3- Guardians and mutalisks attacks. Pretty hard to kill guardians with turrets since they have bigger ranger than turrets.
I spent 8 hours in that single mission. I mined almost every last mineral of the map (some I couldn't reach because I could not build shuttles). t was absolutely awful, but I couldn't stop there. I HAD to be able to say I finished the entire game building nothing but the most basic units of the game.