Here are just a few of the issues in FreeBSD, off the top of my head:
* Differences in glibc versus FreeBSD's libc; easy enough to work around, and many packages already have patches in ports for example.
* FreeBSD has GNU Make as "gmake" - I'll have to symlink this in a temporary directory in PATH at build time (many of Libreboot's sub-projects heavily rely on *GNU* Make)
* Probably have to bootstrap some sort of GCC thing, unless FreeBSD's GNU toolchains are any good.
* Other unknown bugs!