Mercedes has never built an engine with a timing belt. However, that does not mean that they haven't had a catastrophic failure due to a chain failure. The M116 4.2L V-8 engine has a very long chain that changes direction many times. Add plastic guides that grew brittle with age, and you have an engine that requires some monitoring of it's chain and occasional replacment of the upper guides.
Chains will stretch with age, but the beauty is that it's easy to check the timing change and see how much the chain has worn.
Many timing chains live in the engine oil, and they require attention to oil quality and quantity to ensure long life.
The positives are that chains rarely fail in the spectacular fashion belts do, and mostly live much, much, longer than belts. Also, I cannot say for other engines, but on MB models, chain replacement is not much more expensive than belt replacement on other makes. The M111 engine in my C230 seems to require a chain replacement at around 400,000kms (based on other owner experiences) and it's about $450 parts and labour at a dealer. While that is a bit more than the $250-$350 many other four cylinder cars cost, it lasts 2-4 times longer.
I wish our Subie engine had chains, but at 160,000km intervals and about $350, it's not bad for a belt driven engine.