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.