Lower units seem to fail when water mixes with the gear lube or its ran too hard and the gear lube is bad (too thin, no protection)

I would suggest replacing you lower unit oil on a regular schduel and replacing the washers behine the gear lube plug yearly to prevent water from getting into the lower unit.

Power head normally fail when you mix (on 2 strks) if off if you are not premixing oil injection tends to have problems especially on older motors when the temp is cold therefore the oil is thicker and does not mix as much oil with gas

other causes are over reving as john mentioned over triming is a popular cause of this