Can a caster that knows this spell use it to become a Ravener?
<crickets>
I'll take that as a no, then.
Can a caster that knows this spell use it to become a Ravener?
Ravener is a template, so no. Per the PRD: "Polymorph spells cannot be used to assume the form of a creature with a template or an advanced version of a creature."
Yeah there are a lot of templates with undead -- indeed in a way the very type of undead is a template.
So that's a pretty limited spell then...
Ok, i'll take a look at how many untemplated undead there are...
Fortunately there still are several nice undead -- unfortunately you'll never be mistaken for one of them with these spells unless it's by a peasant that had too much alcohol, the number of abilities you do not get is quite large.