I used to have a fine collection of 'dragon pots' when I still had a garden. You should have seen my Japanese maple ...
One problem with them is their tendency to fall over in high winds. Manual watering is an option (I like doing it), but on a sunny, windy roof it has to be done daily. With a drip hose, you can forget about it.
To keep the weight down, mix 'beanbag stuffing' (little loose expanded polystyrene balls) with the compost. To keep the compost moist, always use a mulch: light-coloured gravel is ideal, it won't blow away, reduces heat gain, and it looks good.