Well I have been feeding marrow bones regularly now for 27 years. Never brushed my dogs teeth, never had any dental work, vets always comment on how clean my dogs teeth are, even my really old dogs. They basically gnaw the cartilage off the end of the bones and eat the head, I dont notice them breaking the bone of the shaft.
They are more inclined to break and splinter bones that are thinner walled than the marrow bones. They will crunch and splinter lamb bones so I prefer the good hefty marrow bones. The softer cartilage and porous head of the bones is what is good for teeth, not the bony shank.
The marrow is no problem as my dogs are working dogs and nice and lean.
Just had a look in the mouth of my rising 12 year old cattle dog who has had a couple of bones a week since puppy. Nice white teeth and clean gums, the very tip worn off one canine but no broken teeth other than that and she is a voracious bone chewer.
There are some dogs that dont do well with bones but all mine do fine.