I own Volvos and Benz's. SO here's my two cents worth.
I used to own a 75 Volvo with the B20 engine. It used timings gears (no chain). The OEM gears were synthetic and prone to catastrophic failure, e.g. bent valves etc. They originally used synthetic gears to reduce noise. The after market/OEM steel replacement came along to resolve the failure issue.
I now own a Volvo with the B21 engine. It is the timing belt that you speak of. I have many shade tree mechaninc friends who own Volvos and the conclusion we've all come to is that the belts fail for two reasons. 1) failure to R&R every 50k miles or 2) failure to R&R if they get oil soaked (It is not uncommon for the B21 engine crankcase pressure to blow by the front and rear seals (this mainly happens because folks don't clen or R&R the crankcase ventilation)). R&R of the belt and/or the front seals is not a demanding task. I'm living proof of that and its a job that I don't particulary dread.
Benz chains on V-8s. They certainly learned their lesson on the 3.8 liter US motors (single row vice double row). Again the only thing I can fathom is noise reduction. Double row on everything but those. Chains vs belts? In this case I think chain makes more sense. TO R&R a belt every 50K would require a complete front end tear down to open the space. That's a lot of hours. At least with the chain, if you R&R oil$filter every 3k, inspect your topside every 15-30k, measure chain stretch using the degree difference method (difference between the CAM marks matched against the degrees difference at the balancer, and the listen for not so right noises right at start up (and head them), then it should last at least a 100k. I'm at 130k and 7 degrees. Benz doesn't make cam correction shims(?) for anything past 10 degrees, so I'm due to change mine soon.
The job I do dread is the one before me of R&R of the chain, tensioner, tops rails and/or bottom-side rails and gears for my 117 engine (80 450SEL). Why? Never done it before and I will have to acquire a new tool or two. And there is the added stress of doubt associated with "do I R&R all of the rails as a precaution, or just the phenalic rails? Do I use a chain with links that require peening or side lock? blah blah blah.
Well anyway. Those are my ramblings for the night. FOr our U.S. readers, take the time to thank a vet this weekend. They doing great work out there. I can say that from a position of personal knowledge.
S, J.R. Brown
1986 280GE LWB Anthracite
1980 450SEL Champange
1981 Volvo 240DL UPS Brown