Boot sector programming has always seemed like one of the cooler ways to
play with writing lower level code, but am I wrong to assume it won't
work on systems with UEFI?
It may work with UEFI if it's set to compatibility mode.
That's correct. UEFI firmware will execute a PE image (the same
format that Windows executables use) on a FAT32 file system, and can
access UEFI services to execute a kernel.
The UEFI firmware may support a CSM (compatibility support module) that
can boot a legacy BIOS loader.
This is a great introduction to writing boot sector code! Really brings
back memories. So awesome.