Those horse, water, wind or steam powered line drive machine tools were a real fad for most shops. I guess they were all marketing hype as well. Then there's high carbon cutting tools, and sperm whale cutting oil nobody seems to even want to talk about today.
Neanderthal, those weren't overhyped, they were industry standard but are now archaic.
By way of definition I'd say over hyped or over rated must be a ratio of the amount of hype divided by what happened in reality. So something can be overhyped if the reality was OK, but the hype was huge, but also if there was moderate hype but in reality it was a total flame out.
Coming from academic engineering I can think of a few concepts which may or may not have been hyped in industry but where certainly hyped as research areas. Friction stir welding is one, I've had both professors and school machinists be all excited about friction stir welding and don't seem to know it has been around a long time and is niche at best, and not prototype friendly and not cheap do research on either. That one is personal with me as more than a decade ago I worked really hard helping establish a machine shop in a new university engineering department and could not get the faculty to grasp that an engineering school needs a general machine shop as a starting point before worrying about building a &^%&*% friction stir welding lab.
I'd take Carbide Bob's hexapods as well. As Milland says they definitely have some solid applications but there was a fair bit of academic hype in the 90's from people who enjoy the linear algebra part of robotics more than the building stuff part. I'd agree you can put 3D printing in here too. The reality is fine, many of us use them all the time, but they really aren't improving the way the insane hype has been suggesting now for many years. You also see that companies like 3D Systems, Stratasys and either Desktop Metal or Markforged, have been able to raise insane amounts of capital which they've spend acquiring other companies because that's what you do when you have more capital than revenue potential. Someone I know in the industry mentioned it's hard to be an ethical company in an industry fundamentally built on hype, because how can you market ethically when most of your demand is hype driven, not because a guy has a bunch of parts he needs to make and a 3D printer is the best option.