The exhaust fans on microwaves just recirculate the air. Plus they continuously catch all the grease vapors. We could never keep ours clean so when we did our remodel we put in a stainless steel that exhausts to the outside.

They aren't really exhaust fans. They just recirculate air through charcoal filters. Plus, they make the workspace over the stove too tight.

My microwave hood exhausts to the outside! It just matters how it is installed.

There are some that exhaust outside, I have one and when the temperature drops or the wind blows you can feel the breeze out the vents

Better to have a real exhaust fan over your stove and put the microwave elsewhere in the kitchen. One spot is near the stove, but I have seen them in islands and other well thought out locations in the kitchen. See examples below (hover over photos to expand)