illumos actually has the advantage of a rich set of tooling designed for kernel development (DTrace, mdb, etc.) as well as an ethos of detailed comments -- and it tends to be a bit of a self-selecting community in that it attracts people who value the same. As an example of the adventures of a relative newcomer to illumos, see the excellent writings of Ryan Zezeski, e.g. [1][2][3].

[1] http://zinascii.com/2015/illumos-5498.html

[2] http://zinascii.com/2014/crossed-signals.html

[3] http://zinascii.com/2014/a-posix-queue-implementation.html