The term CPA covers a lot of territory so this statement is a very general one that has a LOT of exceptions - I have found that too large a percentage of the high paying CPA offers I find pay more because they are ripoffs I wouldn't want to promote even if I could easily make a million dollars an hour from them. On the other hand, I participate in several very good converting lead generation programs that pay well because the lead is going to legitimately result in excellent profits for the party paying me. These tend to convert better for me than most Clickbank products.
There is still a lot of junk for sale on Clickbank. There are also very good and truly useful products that convert well on there also. It is just a matter of picking a good one.
Between the two, I would have to go with the lead generation CPA programs I take part in as my preference. I really like the private affiliate programs I take part in better than either of those two though.