I think it is the difficulty of fabrication that keeps teams from using mecanum wheels. Omni wheels are bad enough to make on a mill and lathe.
However... take a look at this. I think a shop with a nice mill, lathe, and tig welder can pop out four of these in 3 days. Or at least mine can
A short description of fabrication: Have a central hub with bearings that two aluminum plates bolt onto. Have slots milled into the outer parts of the plates. Slide 1/4" aluminum shaft, so it goes at 45 degrees (the milled slots do this automatically) and weld one side. Cut off extra with hack saw, repeat. At the end, mill the whole thing flat. Unbolt the side you didnt weld on, slip on the rollers, bolt back together, and poof! You have a mecanum wheel with very little effort.