>advantages
To the extent that *nix-like security depends on obscurity, FreeBSD is another layer removed from Linux. So a bit more obscure = a bit more secure (in that sense).
I found full disk encryption to be a lot easier to set up on FreeBSD than on Linux, and easier to understand. Using GELI. That way you can also post in /g/ threads about full disk encryption and say "U GELI?" I never did that, but I always wanted to.
There's less fragmentation in the BSD world than in the Linux world, so you don't run into the works-on-Debian-but-not-on-Fedora sort of problem as often. The handbook is quite good.
The ports system is nice and is quite current for most common software.
>disadvantages
It's *just* different enough to give you a constant low-grade headache if you're used to Linux.
Flash is a little wonky on Linux. It's a little wonkier on FreeBSD.