From my testing you can start/stop just fine - however you cannot edit a vapp that currently exists (ie, add a new VM to it)
What i wondered about - but cannot test right now is what happends with a VSE device is no datastore exists for it to be created on (they are all disabled) -- This might be why the doc says you cannot poweron. (Would only be issues for vApps with routed vApp networks) - and when no other datastore was enabled for a pVDC.