Generally the limiting factor is the temperature of the core.
The heat generated within the volume of the core is just one factor. To this we have to add ...
- heat generated in the windings
- ambient temperature
- surface area of the core
- core ventilation, is it fan assisted or natural?
The figure of 100 mw/cm3 is therefore a crude guideline, assuming reasonable defaults for all the other factors. It's a good place to start to see whether a design is feasible.
Once a design looks feasible, then you have to go into the detail of what core material, and do some cooling experiments on the shape and size of transformer you will be using, taking account of your maximum local ambient, and the maximum temperature your core materials, wires, insulation, glues, tapes will tolerate for the lifetime you want to specify.