Older Kitchenaid had metal gears, newer ones were plastic. If you are making dough you may break it. We had 2 break, 1 twice.
Then we bought a Hobart from kitchen restock, expensive but no breakage so far.
N50+BUILDUPN50-60 Hobart N50+BUILDUP Mixer, PlanetaryVoltage Options: 100-120/60/1 [Standard]
This was several years ago.