I've read about the FreeBSD Linux emulation layer, but don't know that much about it. What exactly does it provide besides emulation of Linux system calls? How reliable is it in practice?
But you don't need it really. All the usual desktop software is available natively. Firefox, Thunderbird, Chromium, LibreOffice, Transmission, VLC, GIMP, darktable, ioquake3 :-)
You don't need to compile anything (if you don't want to, say, use LibreSSL everywhere), pkgng is excellent.
...games.
Is the syscall emulation layer good enough to run the Steam client and, to pick one at random, Kerbal Space Program?
You need it to run Linux Docker containers on FreeBSD. More on https://wiki.freebsd.org/Docker