Agreed. When I worked in IoT services, I think I only came across 1 instance of a customer with over 10 devices.
I've heard of at least a handful of people that have done this and more this year alone.
Cameras in each room? It's a privacy nightmare and I frankly don't trust my equipment, my network, nor myself to set that up safely. Especially if the cameras I own are any indication of how the rest work (hard-wired default passwords, security flaws, upload of FTP credentials to some Chinese business' servers, constant attempts at leaking data through DNS lookups when regular traffic is blocked, etc).
Thermostats and humidity sensors make sense if each room has it's own heaters/air conditioners, though.
Of course, the first thing I would make sure is that they don't connect to the internet or any wireless network. So I can't imagine the ones the GP is talking about being anything other than a privacy nightmare.
By the way, what happened to data over electricity wires? It's the obvious way to set the home automation stuff.
LAN over electricity wires is a proper pain in the ass. It's not like you can plug in a "router" to one outlet and have network in all others. It only goes to the group that particular outlet is in, so unless your wiring is made in a particular way, which existing houses aren't since it's not an established standard.
One would need a large diversity of products to make it work, what puts it out of reach of any small manufacturer. But why the large home automation sellers insist on their "you must add this huge panel, pass all those wires around, and can't retrofit any new device once the layout is settled" technologies is a mystery to me.
A few phones, a computer or two, TV, a few wifi speakers and a miscellaneous light or temperature monitor. It’s not hard to exceed 10.