I don't think there is enough statistical information anywhere available to make a useful guess about this.
However, there are no inherent limitations in a dvd burner that would make it a worse cd-r burner than a dedicated cd-r only reader.
However, as computer components are very price competitive arena, dvd burner makers have to cut corners where they can. DVD writers use a different type of method to try and maintain and monitor the burn quality on a dvd than cd-r writers do.
So, a good dual format burner does need to implement two types of burning/monitoring implementations, one for dvd-r and another for cd-r. If the manufacturer decides to cut corners on the cd-r part, well... you can guess what can happen.
However, to say for sure that a model X dvd burner is not a good cd-r burner is not easy. I have not seen any tests to prove one way or the other.
I would like to hear about any, if somebody has found some tests on the subject.