The engines are OK IF they're maintained (like anything else really) and unlike older Volvo 4 cyl (red) engines, if its T-belt isn't replaced, it breaks and it'll cost over $3000 to repair the cyl head related damage. This is reported here often and it'll become more common too with newer cars that have 105K mile intervals where it's now a used, second owner car and that owner doesn't know or read his/her maintenance book.
If the serp belt is also neglected, it's breaking can also burn through the plastic cover and destroy the T-belt too. This has been reported several times with S80s (similar belt setup as other FWDs) as many who own them lease them so neglect maintenance.
Another thing I've seen, people neglect replacing thier T-stats (should be done at 70K miles) so the car can be running too hot for a long time and the temp guage only reads slightly higher than normal so the customer doesn't know that it's running too hot. Running the engine too hot for long time periods causes pre-mature ring failure. I've seen a couple 850s where this happened, usually turbo cars.

The 850 engine isn't as much the reason for added costs vs 700s/900s but the rest of the car is. More problems that are costlier to repair occur to all FWD/AWD Volvos compared to older RWD Volvos like 200s/700s/900s. The newer FWDs/AWDs are even more problematic than older 850s, X70s due to their rediculously over-complicated electrical systems.