The message
from contains these words:

It does get a bit hot up there, but no
worse than it would anyway from the solar gain.

Where that idea would fall down in my mind is that the intake air
temperature to the "external" unit would get very high on a hot day,
which reduces the efficiency of the system. And not just a bit, but a
lot.

--
Skipweasel
Pay no attention to that man behind the curtain.