I'm not really opposed to race overlap, but I do prefer the humanocentric position that there should be a reason to add races beyond human. Each race should be somehow alien, if only slightly. Dwarves and elves get a pass because of Tolkien, even though Tolkien elves and dwarves really were somewhat alien if you got into the mythology. I've never dealt with the Warcraft universe, but I could see the orcs getting a similar bye because of it (and I'd rather deal with full orcs than half-orcs; I'm not a huge fan of half-humans).
Basically, I don't want to end up with Star Trek, where 90% of "aliens" were pretty much just humans with random forehead (or ear) prosthetics. Sometimes, they acted as caricatures of certain human traits, but they were more often just as varied. I find that kind of "meh". YMMV, but that could be where some of the resistance to overlap comes in.
And, yes, I get that looking alien can qualify. If you end up with goliath, firbolg, and half-ogre having stats that are 90% identical, then there really isn't a good reason to have them all. Being "ridiculously large and strong without being restricted from normal buildings and adventures" only goes so far before you realize that they could all be one race with the goliath being a world-specific variation that lives in the mountains and tends towards honor, the firbolg is a different world-specific variant that happens to live in the frozen north (or whatever they do with them), and the half-ogre is another variant that happens to have a more primitive culture. Those three might be no different than the Medieval Japanese, Estonians, and Congolese (not checking for historic accurate names), all of which are human.