I couldn't get into Dune. I just couldn't see what the fuss was about. That doesn't mean that Hebert was "overrated", it merely means that it didn't appeal to me. Azimov is widely regarded as one of the greatest science fiction writers of all time. If your works are still being read 70 years later, you are obviously doing something right.
Not everyone is going to like every author. It's not just with science fiction authors either. Dickens is beloved by a great many people, but there are people who scratch their heads and wonder what people see in Dickens. They aren't wrong to not like Dickens, it is a matter of personal taste.
Asimov did write less science fiction in the 60's and 70's, but he was also writing an amazing number of science books during this period. Here's a link to Asimov's bibliography, sorted by year.
http://www.asimovreviews.net/Numerical.html