I/O need more maintenance than OBs. Std replacement items on I/O, that are unique from OBs are circulation pumps, manifolds, risers, gimbal bearings, U-joints, driveshaft/shiftshaft/exhaust bellows, water hoses and fan belts.
They also have all the same maintenance items as outboards, such as impellers, thermostats, spark plugs, gearoil, filters etc.
Also, it seems like I/Os need to have replacement starters, alternators, solenoids and the other stuff we used to replace on 70's American cars. Not sure why.
In salt water, the maintenance items need more often replacement. OBs are easier to trailer due to lighter weight. OBs are easier to work on, on a trailer. I/Os are easier to work on, in the water.
On the plus side, I/Os get better fuel economy than 2 cycle carb outboards. That may not be a factor, if you compare an I/O to a 4 cycle or DI outboard.
One more thing, if you keep the I/O boat in a salt water slip, be prepared for oil filters, timing covers and oil pans to rust thru. It will take a few years, but it happens.
Not sure about you, but I don't enjoy working on any motor as much as I used to.