Hi!
Yes, I noted that about windows task manager, so I monitored the gpu usage with GPU-Z, and show the same behavior.
About the non so taxing project, I processing it with 8 worker on the same machine to demand the hardware to the max: the nvidia's gpu work to 100% and they are very loud (max fan speed), but the amd gpu only make loud on the second half of "match photos" with 100% gpu usage on windows task manager and the performance/time improves considerably, see the table below:

(I know the gpu's are used only on Match photos and build depth maps phases)
Now, see the time in "build depth maps": just a small improvement over the nvidia 1070. I have also tried to use only the 7900XT, and in phase "build depth maps" it has the same low gpu utilization (ckecked on GPU-Z) and the fans do not make any noise

. while the nvidia cards at this stage are at 100% and with the fans at maximum speed.
EDIT:
Now I tried with a bigger project, 7300 photos. Here the utilization:
