With 1u the clearence for the rails is taken away from the long side of the module, e.g., you have less space to work with compared to a 3u, given the same size front plate. You cannot just give a 3u module a quarter turn and declare it a 1u, you will need a new layout to fit it in the narrow space. I guess that's why my 1u uO_C was so much more expensive than the 3u version. Moreover, the smaller the module the less functionality will fit in while much of the cost will not shrink accordingly.
I was thinking of a more or less self sufficient rack here. But given a special use case it does make sense, I can see that.