Why would the mount disprupt air flow?
Because a firewall mount by default needs a firewall, and walls block airflow, especially perpendicular ones
And since all of this is one vs the other topic, beam mounts do not need a firewall - so no wall, no potential blockage.
Looking at this in its simplest terms, the task is to link the thrust unit to the lifting surface, and a straight line provided by a beam between the two undoubtedly is the simplest method. An RC style firewall mount adds complexity and personally I would prefer to mount directly off the engines back plate to the firewall 'if' I went for that system and that is something that is not possible with most four strokes because their back plate is obstructed.
Igor Burger I believe went for the direct back plate system with his 'Max' model as do some RC fliers with their indirect modified and combined back plate/ radial mount systems.
Its not that I don't like RC style mounts, its just that I prefer it differently.
Cheers.