Uber to Zaventem airport - drop-off point

I would imagine they would just drop you off at the general drop off where taxis and normal cars go. When you order an Uber the driver will know where you are going so he would not pick you up if there was any limitation, but I am almost certain there is no problem dropping you off.

That aside, Taxi Vert have actually quite some competitive prices going to the airport. 35 euro or so if I recall correctly from central Brussels. It is a fixed price and if I recall correctly it was actually cheaper than Uber.