I think one of the biggest problems with the home brew xenos races, is that there is no rules support to back them up.
If I were to play space marines, there are Ultramrines, Raven Guard, Imperial Fists, White Scars, Iron Hands, Salamanders, Black Templears, Blood Angels, Dark Angels, Space Wolves, and Grey Knights to choose from, all with their own rules (and gear, in some cases).
Now, if I wanted to play Eldar, there are different crafworlds, but no real divergent rules for units. The same goes for dark eldar, orks, tau, nids, and necrons. Aside from some minor force org shifting using special characters (like a Warboss) or unlock characters (like a haemonculus) there is little variation to army design.
So if I want to make a renegade Tau mercenary force (WIP), there isn't much to differentiate them from a cadre from Tau itself. The lack of real differences makes me more likely to buy SM and customize to taste.