FreeBSD is an operating system. Linux, strictly speaking, is just a kernel, but the two systems are not too much different and in performance, at least, we can say they are comparable.
Open source software works across the two platforms so there are no cross-platform issues.
Reliance on professional services support will not be too high as web applications can get help from the community.
How did this trend develop? FreeBSD best practices? Are there any success stories?