As stated by others there are way too many factors to do an accurate correlation. The majority of cams fail due to human error. You also need to look at the quality of the machine work, pushrods, valves etc... Way too many things to point the blame in any one direction.
One one hand it would make sense that there are more failures today then say 5 or 10 years ago since there are more parts coming from different areas. 5 or 10 years ago the majority of street guys were still using factory components. Now more, and more are using aftermarket components, and mixing and matching of brands. Block from one source, heads from another, parts of the valve train from who ever had the best deal, some machine work from one guy, some from another, and so on. Bottom line in a controlled environment cam failures are very small.
On the other hand it's normally flat tappet cams that fail. I would think the number of people using flat tappet cams has got to be declining with all of the standard, and hydraulic roller stuff around.
The majority of cam failures I've ever had were due to an unstable valve train. Once getting a look at it on the Spin-Tron you'd be scared of the valve train.