Fixed splitters are usually recommended for shop made zero clearance inserts,
are effectively fixed height riving knives, and I've heard no reports that they
are ineffective or dangerous. Since they are fixed height, they will sometimes
be higher than the blade, sometimes lower.

Putting these observations together, I would theorize that a riving knife that
was cut down to the same height or slightly shorter than the blade extension
would be better than a fixed height splitter on a zero clearance insert.



a minor point here- a factory splitter or riving knife being attached
to the trunnion and passing through the throat insert needs a slot in
said insert. that slot does weaken the insert, making it less rigid.