Top airflow tanks have their problems, but they've improved a lot over the years. When OBS released the Engine, then the Engine Nano, I thought they were going to make many more really nice tanks. Afaik they got eclipsed by the other brands designs for RTA's. It's too bad really, they were onto something good. The Nano was the RTA that changed my opinion of top AF RTA's. The ability to deliver excellent flavor, good airflow, and easy wicking, while providing a highly leak-resistant experience, was a huge improvement from most previous attempts at the time.
I guess that's why you still see them around occasionally, in setup pics, etc.. There are a lot of people who still love them for what they are and how they perform. Glad I was able to get them when I did.