From what I know about UEFI there could be many reasons why it's not working. As these are older machines it could be a buggy version of UEFI on those systems. And apparently UEFI can also come in 32 or 64 bit flavors. If I recall correctly a 32 bit UEFI could cause problems when trying to boot a 64 bit FreeBSD. The Windows bootloader is probably more forgiving and capable of working around those issues. After all, both HP and Dell pay Microsoft huge sums of money.