I think its just subjective but i also think folks that would prefer "uniformity " are leaning more towards aspects like anatomical proportions, anatomical realism, etc. For me personally its this above all else. There is no denying that the kruleboyz, just as the new stormcast, have taking a much different approach to model design.
Im a very detailed person so it's very easy for me to spot these not so subtle differences across the Orruk warclans models. Things like ridiculously big heads, teeth that wouldn't allow you to even close their mouths let alone put food in, over the top muscle tone, etc.
Yes a unified paint scheme helps but for some its way more than that. Again it's all subjective.