Can it be used to fix a hard brick too?
Of course it won't fix broken hardware.
It can fix every brick situation that is the result of incorrect contents of the internal eMMC. You can backup and restore the Tegra boot config table, the bootloader, partition table, kernels, recoveries, and all other partitions. As long as you can turn it on and activate the NVidia recovery mode holding down volume-up, which shows up as NVIDIA APX device on the connected PC under Windows, AND you have obtained the needed files *beforehand*, NVFlash can fix it.