To me, a true single inboard with a prop shaft is a great marine propulsion system. With a single keel-cooled inboard diesel and a shaft that didn't change directions a bunch of times, and with the addition of thrusters, it would be the power of choice for a larger vessel. My problems with the I/O arrangement (based on hard personal experience) are the gearing complexity of the drive unit, and the fact that so many of the propulsive mechanicals are always submerged. I also personally believe that Mercruiser is of marginal quality; more prone to failure than Volvo Penta, Kodiak, Crusader, and some of the other makes. Despite fastidious maintenance I had to have my Alpha 1 outdrive rebuilt 2x and the Mercuriser 470 motor rebuilt once during my 5 year ownership of a trailered boat which never stayed in the water more than 2 weeks at a time, and mostly lived on the trailer. Outboards have been much more reliable in my experience. I have not had a single problem with any of the 5 (new) Honda outboards I've owned., had few/minor problems with a used Johnson 70, had a catastrophic failure of a Johnson 40 (which had sat unused for decades and the owner had made no guarantees), and no real problems with a Merc 15 kicker, or with a Tohatsu 6 kicker. I also had good service from a couple of older Evirudes (20 nd 9.9) which had previously been sitting for long periods of time and whose carbs I had rebuilt. I will say quite clearly that the 4-stroke motors have been much more enjoyable to own than the 2-stroke despite being much heavier.