What are some functional differences between orchid mixes?

[Daenerys]

Something I was wondering about. What's important to having a good orchid mix, beyond the size of the bark?

I ask because I bought some 'paph and phrag' mix from repotme.com, which I used on my paphs a few months ago. But then I also used it when repotting my oncidiums and den victoria-reginae (It was the only fine-grained mix I had).

Now, all these orchids seem to have thin roots... but is there any reason why a mix formulated for oncidium would be better than what I used? Or are the differences between such mixes mostly cosmetic?