It seems to be that the SoCs are limited in how much total power they can draw which means they have to allocate it between the CPU and GPU cores and therefore cannot run both at maximum. This might be a hard limit of the design or Apple being too conservative in their power settings in order to keep temperatures within a certain range (sub-70 degrees Celsius).
Hopefully it is the latter case, as Apple could then release new firmware that allows more aggressive power draw since the cooling system (especially for the Ultra) has plenty of headroom.
This seems to be a hardware limitation and not a software one. That being said, they have noted that Apple has yet to release updated versions of Logic and Final Cut that do better exploit the Ultra's extra cores (review units were shipped with these updated Apps, but they are not yet available via App Store to end-users).
If the issue is the design, then yes it could become an issue presuming Apple improves the design in future M series SoCs to be able to use more of the total power available to them.
If the issue is Apple being too conservative in the power configuration firmware, that should be able to be adjusted on the M1 family via future firmware releases to allow more power in exchange for more heat generation (which means more fan noise).