Nanotubes and graphene themselves had a deep hype cycle. They're super strong... as long as they're atomically perfect. The second bit just kind of got left out of the popsci stuff circa-2009.

The biggest thing about technology nobody talks about is manufacturing. The physics was there to design a modern GPU in WWII, it's just that reliably making a thing with few-hundred-atom switches starting with 1940's tools is a very hard problem. The correct approach to do so wasn't even clear until the 70's or so, and then it took many decades of finetuning and building up ever larger and more expensive fabs that can print features that small in environments that perfect.

There's no clear way to place individual atoms of a large object at reasonable price right now. Maybe some kind of biotech will make it possible eventually, or just scaling up current atomic manipulation techniques a lot, but for now CNTs are but high-tech asbestos.