It is fine to feed a sub-panel to support the HVAC system in the attic as we do this all the time. However, you need to make sure the feeder is properly sized for the application…this is important.
In regards to the mounting it would not be a huge concern if the unit is mounted securly to the web of that truss system…if the top of that panel is the only supporting location then you have to inform them it needs to be supported from the bottom as well…IMO it is not secure if only one screw is being used.
Now…the other issues is supporting the wires as they leave the enclosure…if they turn and go to the web of the truss and are supported with staples and so on then that is fine.
Make sure it is being fed with (4) wire…obviously, and that it is grounded and bonded correctly as usual…and if it is fed with SE…and done prior to lets say 1996…you can inform them that (4) wire is used today to feed sub panels but in fact they do not have to replace it…as it was code at the time of inspection…but I find it important to make them aware of the change to (4) wire because it was proven to be safer in reducing objectionable current on the neutral and so on…
Other than that…I would not get too pushy on this…many electricians install a SUB-Panel in the attic to feed the air handler if it is a larger one that required a 60A and 30A…just make sure they sized the feeder correctly.
UPDATE- Greg makes a GREAT point however…may not have any 120V needs at this location…clearance and location is key…and how it is labeled.