I have a over the range microwave/vent hood in a rental I own. After eight years the microwave failed (an electrical burning smell whenever it's running). It's a Kenmore.
So I bought a replacement (Kenmore) and went there last night to install it. The mounting hardware has changed so I had to put the new wall bracket on. They moved the top mounting holes (that secure it to the cabinet above) forward and out. Now the mounting holes have to go through the front molding of the cabinets (which I think is only glued or nailed on) so I reinforced it with L brackets. I also put in flat plate brackets on top and screwed them to the bottom of the cabinet then put the mounting bolts through the plates. One of the mounting bolts is now so far to the right that I had to trim the outsider right molding a tad so the bolt could go through the hole.
I don't know why they felt compelled to move the mounting holes to such bad locations. Usually those top cabinets hiding the vent pipe are pretty shallow.
Unrelated to the installation of the new unit, I realized that the contractor that remodeled the kitchen and put in the original unit had not rotated the squirrel cage blower assembly to the position for top venting (it comes set up for rear venting). So while the vent pipe through the roof was connected to the top of the microwave/vent hood unit, the blower was blowing directly against the sheet metal of the back of the unit for eight years, essentially doing nothing. Since I didn't live there I never noticed it and no tenant ever complained about lack of venting.