Unfortunately, my experience has been that it's the endless cycling with customer requirements that takes all the time.

The hardware is generally manageable quickly for me. Even th firmware isn't terrible.

And then you deploy it.

And the customer realizes it needs to do Q and that they asked it to do X and it doesn't need to do that and we need to take another $0.50 out of the BOM and ...

And then 4 years pass and you still don't have any real revenue.

There is a reason why so many places demand NRE of $1 million minimum and THEN they'll talk about designing your project.

The big gap isn't scaling. It's the scaling PATH. You need to have a viable and profitable path all along AS you scale from tens to thousands to millions. And those projects are VERY rare.

See: BeatBuddy for interesting post mortem and note that AI wouldn't really have helped them much at all.