I've owned both as well.
With the I/O combinations figuring out maintenance is a bit easier than some of the newer OBs. Regular maintenance items like alternators, starters, distributors, cooling systems etc are typically easier to access (subject to the design of the boat's engine access). While an OB is smaller, the parts do tend to fit together like a jigsaw puzzle and in some cases require specialized tools for removal and replacement. Generally speaking, most newer I/O engines use small and large block GM engines and there tends to be a much larger (and cheaper) variety of parts available where the OBs tend to be proprietary and have a much higher price (for example, a solenoid relay on a Mercruiser is about $25 vs. $85 for a Yamaha (same exact part with a proprietary mounting bracket).
I tend to think of I/O motor/drive combinations as modular with OB's more generally fixed (not exactly true but sort of fits). A bit easier to replace/source any modules that need to be replaced on an I/O (upper or lower drive, pumps, cooling, exhaust, electrical and even the block typically bolt off and replace easier).
But...all that said the OB's are a nice sealed unit that don't get as much exposure to the environment. Pull the cover off a well cared for OB and they can look brand new where an I/O tends to get wet. I'd tend to say an OB requires fewer repairs over time than an I/O but when major items come up, the OB is more expensive.