I might be mistaken, but it seems the firmware (UEFI) needs to load some stuff from the disk to be able to do anything really interesting, like loading some advanced tools or loading the OS. Yet firmware infections are often associated with the concept of "persistent infections", where wiping the disk won't help. But what can an infected firmware do on its own, after you have wiped the whole disk? Obviously malicious firmware can load all sorts of malicious stuff, but first it must find that stuff somewhere. Maybe the firmware memory is large enough to contain malicious code to inject somewhere later in the boot process, but I'm not sure there is enough memory available to do anything interesting that actually works.
So what can a malicious firmware after the disk has been totally wiped (every partition has been overwritten) and another clean OS has been installed?