Sure the units are similar and shouldn't look vastly different, but it would be nice for some to be easier to distinguish.
In my case it's not usually a matter of separating them in army setup. As has been said it's easy enough to double click to separate them.
It's more a matter of buying the wrong ones in the first place. I accidentally click and buy ten of the wrong one and don't notice because they look similar.
I have one issue with units looking like each other. Very often if I play LA Pythium, I'll buy an assassin instead of a serpent priest. Which is really a mistake I don't want to do. And, I know they are a bit different, but the assassin looks like a priest imo.
I'm going to go on the record as saying that I don't really think units look too much like each other. There is the rare unit that looks almost identical to another unit almost exactly like it, but as others have said these rare units are basically interchangeable. But for the most part, even similar units are sufficiently different-looking so as not to be too hard to tell apart. And I say this as someone who would not be physically capable of reading a thing on their screen without glasses.