I use FreeBSD more than 8 years. When SWSoft release it's plesk version 8.x I paid licences to use them on FreeBSD, previously Plesk installation was packaged for Freebee and it was simply to install them without any uubeer super-trouper installer downloaders and other buggy shits, as additionaly I wanted to make changes in ports tree, everything goes well and I was able to use portupgrade etc. Now Plesk installer uses complex rubby scripts when elemetary operations such as indexig ports on rubby scripts slows down the system, install takes so long time. I installed Plesk manualy through their ports dir, it was faster. Plesk starting versions 8.2 was so buggy. All versions are made for FreeBSD ports that was fresh at the develpment time, but development goes for long time and when plesk releases, integrated ports tree are very old. If I choose to install Plesk on existent ports tree, install drops out many errors. No one version of Plesk 9.x was successfully completed without errors. As I saw, the errrors on new versions of Plesk are the same, rubby & portugrede buggs. SWSoft offered me to buy Plesks SUS, after that I will be able to use latest Plesk versions. The questions is why I need to pay for nothing? Install buggs from previous versions aren't solved, no support for x64 architecture. Experimentaly I installed i386 version of Plesk on FreeBSD 7.x amd64 architecture under i386 32bit compatible libraries, taking these libraries from FreeBEE 6.x versions and some were existent, but Plesk was partially functional because it uses libpam_plesk.so module to authenticate DrWeb antivirus and I wouldn't be able to use it cause of incompatible architectures.
At the point of view of development it's is not too hard to develop FreeBSD x64 version of Plesk. I think that simply SWSofts developers are so lazy and with the small experience in unix (not linux) such as software packaging, ports tree, modules etc. Why did cpanel supports FreeBSD also open source ISPCONFIG also supports freebsd, their jails subsystem...
The main thing to use FreeBSD is it's stability, scallable platform in other words - best server for the web versus linux with their frameworks