My Volvo has never let me down, but it does require more maintenance.
Curious as to what additional maintenance yours requires.
We've owned 4 Volvo's - 2 MD20xx series, and 2 D2-xx series - and none of them have required more than normal routine maintenance expected from any engine, none have given us any problems, and they are all easy to work on. The one Yanmar we own is a different story - the raw water pump is mounted backwards making it completely inaccessible without removing the entire pump completely, some of the hoses are routed in ways that chafe and are also inaccessible, the exhaust elbow design is a nightmare waiting to happen, and there are other ways it is illogical and compares unfavorably to Volvo. It also has only required routine maintenance, albeit more difficult to perform, and has not given any real problems. We owned 2 Kubota-based engines, and these are the most rugged, easiest to maintain, logical engines I've dealt with.
As for parts, Volvo is no more expensive than Yanmar. Certain parts cost more, but others go the other way.
For example, the following prices are $Volvo/$Yanmar for comparing the MD2030 to the 3YM30 (these seemed to be equivalent engines used in similar boats).
Oil filter: $12/$7
Fuel filter: $16/$5
Head gasket: $96/$78
Mixing elbow: $248/$186
Fresh H2O pump: $286/$318
Raw H2O pump: $258/$238
Injector: $239 ($137 exchanged)/$193 (no exchange price listed)
Injector pump: $864/$2,300
Heat exchanger: $517/$1,400
If you ignore the oil and fuel filters (which are readily available for the Volvo engine from other manufacturers in most autoparts stores for far less) then the pricing of parts seems to be very close between the two manufacturers until you need something like a heat exchanger or injector pump. Then the Yanmar pricing is eye-popping.