Compared to what else is available now, they just aren't very high on the bang for the buck list anymore.
They have a few quirks that some other choices don't........ And IMO, they don't do anything "better" than other current choices.

That, along with them being about the worst for header fitment/spark plug access in a regular door car doesn't help the sales volume.

They can make good power and are well made......I just feel for whatever the application.....some other head would make a better option.