Overfilling a motor with oil will indeed create a froth of oil that does not pump well.... that can lead to oil starvation and a seized bearing...... this is not the greatest risk resulting from too much oil.
If the cases contain too much oil, it will eventually get pulled through the EGR valve and will be dumped into the air box, behind the filter. This always happens to some small degree; the harder you run the motor at the greatest lean angles possible, the more you will see it. That's why there is that litlle bottle hanging off the lowest point of the air box, so you can see it and keep it out of the engine. It's only meant to catch a tiny amount of effectively condensate.
If you are seriously over filled -- and this is greater than a quart over the upper limit -- you are at risk of causing hydraulic lock. That would be bad. Catastrophically bad.
When the EGR (exhasut gass recirculation) valve dumps the engine case gasses back into the air box, so the engine can burn off the oil fog rather than vent it to the atmosphere, that's a good thing for people, kittens, whales, etc. The fog occurs because the rings cannot have a perfect seal to the cylinder walls.... it's unavoidable.
When that same valve sucks up liquid oil and pumps that into the intake -- liquids do not compress. If the volume of liquid is high enough, it will stop the piston from moving up to complete the compression stroke. A 13:1 4 cylinder 600 cc motor has a very small amount of volume above the pistons at top dead center.
600/4 = 150 (volume of one cylinder at bottom dead center)
150/13 = 11.5 cc (volume of one cylinder at top dead center)
Not only do you have the fuel/air mixture in that volume... there is the tip of the spark plug, and those pesky valves that are SUPPOSED to get out of the way before the piston gets there.... extra drag, even higher than expected compression, those are bad things as well.
If you do manage to get 11 + cc of oil into a cylinder... bent valves, bent rods, damaged crankshafts, totalled motors......... usually this involves a big puff of smoke, a 'bang' and quite possibly a locked rear wheel -- or a huge amount of oil under a locked wheel.