Can it overwrite the MBR on a Safe Boot / UFEI machine?
No word yet. It has just been detected and the coders are still examining it.
Best to have backups ready now just in case.
If this happens you take the infected hard drive and install it as a slave (or non bootable) on a clean desktop computer. Pull off all necessary files you want to keep. Wipe clean the infected hard drive and reuse it
No. The EFI uses a completely different partition table, GPT.
On computers that support booting directly from GPT disks in EFI mode, there may or may not be an MBR at all. Some modern disk utilities, however, will refuse to re-partition the drive if the MBR does not exist. (You would have to erase the entire partition table, and start over)
Otherwise, the disk usually contains a stub MBR whose purpose is to store a custom bootloader for the rest of the GPT disk, in exactly the same way Dynamic Disk Overlays (Kroll) were used in the 90s for large hard drives to overcome various capacity limits.