I repowered just before cat converters were introduced to boat engines, so I cannot say what has happened since then. Mercs have an excellent Guardian System which I really like. When the raw water pressure/flow gets low, the system reduces rpms to a max of 2500. So rather then overheating the engine if there is an impeller or intake issue, it gives an alarm and forces a lower speed. There are other scenarios that would activate the guardian system.
Also they went to a dry flange exhaust system. There are no water passages at the manifold to riser/extension mating surfaces. Traditionally these would have to be disassembled periodically and cleaner, possibly replaced. With the dry flange design, that problem completely went away. The only upgrade I would otherwise like is a distributorless ignition.
I know Volvo has gone to an aluminum block, which may be excellent. I think this is a great idea. However Volvo has put out some very poor products in the past and I would not buy a new model engine until it had been out for a few years. As to parts prices, they are 2x to 3x what Merc charges for many items, and Volvo often designs things so that the parts are not available aftermarket. They change their parts designs on a regular basis so Sierra and other manufacturers do not make replacement parts, as the numbers sold would not be profitable. A company like that I will not support. I worked as a marine mechanic for 20 years, and I have extensive experience with Volvo.