Why is combining it with a microwave inherently bad? As long as there is an air channel to the exterior, the critical thing should be air volume over time, no? (CFM or m^3/minute or whatever)

I am assuming that effectively all microwaves move less air than dedicated range hood. From cursory Googling, microwaves vent at 300cfm to 400cfm, while dedicated range hoods can move 600cfm to 2,000cfm.

As the other person said most microwave vents barely move any air and in addition most are recirculating the air not venting it.