I have (2) Yanmar
SD20 saildrives, 9 years old, 1700 hours. As you surmised, there is normal maintenance, mine get diver cleaned monthly, I change zincs in the water
every 6 months (per Yanmar recommendations) and change oil
and lower seals
every haul out
for bottom paint
(2-3 year intervals). We check oil
regularly when checking engine
oil. Any sign of water in the oil (milky color) could cause an unanticipated haul out
to replace seals (not happened yet!).
Saildrives are 'scary' from the fact that your transmission
is hanging in the water, hence requires preventative maintenance to insure it stays working.
There is obvious trade-off, but you gain the flexibility on where to put the engine
and space. The prop is more efficient as it's parallel with the waterline vs. pointing down.
No system is perfect, but if you maintain them as designed, they'll last a long time.