Day often has many wise words of wisdom. One of them that has particularly stuck with me is: “A good unit composition is not about what you can add to it to make it better; it’s about what you can take away.”
What does this mean? Wouldn’t having a few Thors sprinkled in to that Marine/Marauder/Medivac army mean you could trample forcefields? Why wouldn’t I want Roach/Hydra for DPS and Mutalisks for harass with a lot of Swarm Hosts to siege my opponent?
The problem with adding too many different units to a unit composition is that you first have to invest the money into the infrastructure to produce that unit. And then once you get the ball rolling, you also have to commit into the upgrades needed to take that unit to the late game.
Keeping it simple requires less structures, less upgrades, and less time to get off the ground. Take Roach/Hydra for example. It requires only 2 structures: Roach Warren and Hydralisk Den to produce the unit composition. Then, the only upgrades you require are the Roach and Hydralisk specific upgrades, and Ground Ranged Attack/Carapace upgrades. Very easy to mass produce without having to invest in basically anything.
Same thing with Chargelot/Archon. It only requires a Twilight Council and Templar Archives, a bunch of Gateways, and Ground Weapons/Armor upgrades. Likewise with Marine/Marauder/Medivac. You need a Reactored Starport, a bunch of Barracks, upgrades from the Barracks Tech labs, and Infantry Attack/Armor upgrades.
Keep your unit compositions simple and you’ll be able to invest a lot more money into the actual units and upgrades you want to have a massive, upgraded, lean army.