I've just skimmed through the PDF of "Codex of the Black Sun". Wow, this is good stuff! Still waiting on the hard copy but in the meantime there's a lot of reading to be done. This is an excellent book.
The number of spells per spell level for arcanists is roughly comparable to those of B/X magic-users. The number of spells per spell level for the magister classes is roughly comparable to those of B/X clerics.
Wondering if you can create undead by having a Shadow possess a corpse.
I'd allow that. Similarly, create empty clone bodies and put Shadows into them.
I am playing Roll20 Campaign where we just introduced the Codex.