A couple of good friends of mine spun out hardware start-ups over the past five years. Even though I'm a data/software guy, I helped out with both, up to and including soldering components onto demo boards. Since I've a couple of kids, mortgage, temperamental automobile etc. etc. I wasn't in the position to work for either startup for low salary/high equity though, and I'm kind of glad I didn't TBH.
The first startup should be way bigger and well known than it is, but the product/market fit isn't too good and marketing in general is poorly executed. This startup looks at electro-muscular stimulation to help out diabetic and pre-diabetic folks with getting some exercise (and folks with mobility/joint issues). To be honest I can't see this company surviving another five years unless there's some radical shake up in the way it's marketed, which is as a consumer device rather than a medical device.
The second startup is a spin out from a medical devices incubator, and is a real niche market - basically nerve implants to manage chronic pain. The target market is something ridiculously small, on the order of a handful of people per 100k who may benefit from it. At the moment, the only thing keeping this startup afloat seems to be generous research grants, can't really see how they can stay going unless the overall plan is to drive some hype and hope to get acquired by some bigger fish.