@varsey @dheiny, I’ll jump in if I may.
Firstly, this setup would work for buoyant flow and other analysis types as well. As a side note, there are several ways to use this combination, each bringing certain benefits (but that’s a different topic).
Regarding the second paragraph, there are two issues. The more obvious is that, if you know the values of pressure at the two outlets, you could specify those values at Pressure outlet definition. This will result in a valid setup with, in the special case of the simulation you provided, different mass flows at the outlets.
The less trivial case, as you mentioned, is when the exact pressure values are not known at the outlets. If you replace outlet conditions with Set gradient to zero for both velocity and pressure (you should use Custom BC for that), you will see the simulation crashes at the first iteration. Since no pressure values are defined in the domain, the incompressible solver cannot fully constrain the system of equations.
To move forward, you could assume the flow conditions are fully known at the inlet, i.e. both pressure and velocity. Use a Custom BC to set fixed values for both at the inlet, and leave outlets to be zero gradient. Now the system is well-constrained, and your simulation will successfully run. Mass flow at the outlet now depends on the features of your geometry.