Here is an objection to the pterosaur to bird theory:
To derive birds from pterosaurs would require some pterosaur to give rise to terrestrial maniraptors, because non-paraves maniraptorans are terrestrial, and the bird lineage evolved within the maniraptorans
It must first be recognized that the so-called "non-paraves maniraptorans" are secondarily flightless and come after the most basal paraves*.The pterosaur to bird theory proposes that flying pterosaurs gave rise to flying basal paraves.
In the dinosaur to bird theory, the so-called "non-paraves maniraptors" are incorrectly considered to come before (ancestral to) basal paraves.
However when we work with the idea that they are secondarily flightless (descended from basal paraves) then the objection above would not be relevant.
Pterosaurs did not give rise to terrestrial maniraptors.
* they are actually members of paraves