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.