The main thing, Mark, is that photography of people in UAE without their permission is illegal. And there has been quite a lot of stuff about this recently in the local media. Dashboard cams have been the subject of a lot of debate.
So too many folk flashing long lensed cameras from a roof (for whatever purpose) is likely to attract some attention and maybe even a cease & desist !! The Sheraton web site actually advertising spotter photography opportunities was frankly asking for trouble.
Photography of objects (but not military installations, Palaces, and bridges) should be OK. There are loads of places in Deira and Mamzaar districts where you could, discretely, pursue your hobby.