There are so many units for all races that more or less blend together in that their combat roles overlap. Goliaths and Diamondbacks are the only units whose combat role isn't overshadowed by other melee units - Wraiths are less useful than Vikings for anti air and less useful than Banshees for cloaked ground harassment. HERCs, Firebats, Predators, and Vultures are all overshadowed by Hellions and Hellbats. Spectres are actually stronger than Ghosts in terms of brute force but much weaker in terms of utility, but their roles are so similar that I wouldn't include both of them simultaneously (you'll note that blizzard themselves avoided having both of them controllable at the same time and made them exclusive). The same can be said about Ravens (offensive) vs Science Vessels (defensive/utility). Hercules are really just straight up better than Medivacs since you have Medics in the campaign already.
Thankfully the overlap between zerg units is less noticeable because the strains were exclusive and built to be more about playstyle (do I want beefier units that never die, or fast units that do a lot of damage, or units with utility, or some combination thereof?).
Protoss are more akin to terran just because there are so many units, but again, they are exclusive. You must choose between three units of a similar combat role, rather than attempting to find a way to balance the combinations of all three.
In the end, less is more. Start with just the bare-bones combat roles, and then add more as are necessary, with a focus on avoiding making units gimmicky and only useful for one or two maps. Obviously some units (marines, zealots) are useful in almost all cases, and some other units (banshees, phoenixes) are more specialised, but that shouldn't stop you from retooling existing units or designing brand new ones that are useful in many circumstances.
There are so many units for all races that more or less blend together in that their combat roles overlap. Goliaths and Diamondbacks are the only units whose combat role isn't overshadowed by other melee units - Wraiths are less useful than Vikings for anti air and less useful than Banshees for cloaked ground harassment. HERCs, Firebats, Predators, and Vultures are all overshadowed by Hellions and Hellbats. Spectres are actually stronger than Ghosts in terms of brute force but much weaker in terms of utility, but their roles are so similar that I wouldn't include both of them simultaneously (you'll note that blizzard themselves avoided having both of them controllable at the same time and made them exclusive). The same can be said about Ravens (offensive) vs Science Vessels (defensive/utility). Hercules are really just straight up better than Medivacs since you have Medics in the campaign already.
Thankfully the overlap between zerg units is less noticeable because the strains were exclusive and built to be more about playstyle (do I want beefier units that never die, or fast units that do a lot of damage, or units with utility, or some combination thereof?).
Protoss are more akin to terran just because there are so many units, but again, they are exclusive. You must choose between three units of a similar combat role, rather than attempting to find a way to balance the combinations of all three.
In the end, less is more. Start with just the bare-bones combat roles, and then add more as are necessary, with a focus on avoiding making units gimmicky and only useful for one or two maps. Obviously some units (marines, zealots) are useful in almost all cases, and some other units (banshees, phoenixes) are more specialised, but that shouldn't stop you from retooling existing units or designing brand new ones that are useful in many circumstances.
My YouTube | My SoundCloud | My Twitter