It's a very interesting concept. I had heard about it in parts of Belgium; where are you based? Here (france), we use drinkable water to pee into.
It's all over the country for newly built housing. Subsidised and obligatory (for new houses) in some regions. Where I lived before, in Antwerp, the apartment had the same kind of system.
The latest is green roofs, but that isn't catching as fast. Fear of leaking and price, I think.
There's also a serious effort to get our aquifers refilled. And Nestlé can't buy sources here. They already owned a few and have been warned they'll run into the monopoly law if they didn't stop immediately. There is some effort to stop multinationals from buying water sources in the EU, but I don't know where that stands atm.
But we're stubborn, so there still are housings being built without rainwater systems. Like my neighbours. They have a swimming pond (latest hype, instead of a pool), but no rainwater system to feed it. I think the reason is that you can't see the rainwater system. No bragging rights there. I know they'll regret that in the future, as feeding drinking water in the pond will destabilise the biology of it. Too hard water (around 30° German hardness) isn't suited for the swamp life that's native.
Is it a mechanical valve? Where is the tank? I suppose you have specific plumbing for toilets. Here toilets are just connected to anywhere there's water.
I have no idea. Haven't even seen the installation, as it's somewhere just below the roof. There must be electrical pumps to get the water to that level. But I've developed fear of heights getting old. No ladders or walking on roofs for me. There's a company that does the yearly service. Mainly cleaning the bottom of the tank. All inclusive (also any repair) it amounts to 105€ yearly. I wouldn't want to do it myself. It's a dirty and somewhat dangerous job.
My grandfather's house, which is almost a century old, also has a big rainwater tank in the attic. No valves, no pumps. That does mean there's almost no pressure and some flush tanks need to be modded. You just need to make the little hole in the venturi a tiny bit larger. The hard part is the "tiny"
bit. A little too large and you can throw it away.
Only thing is, the tank never ran empty before, but the last ten years or so, it runs dry during summer. Then you just close one tap and open another. Simple, but that system is no longer permitted, as you could force rainwater into the drinking water. At least, in theory. What happens is that the tank spills over, as the drinking water has around 1,5 bar pressure and the rainwater only 0,1 bar. If you don't notice, it'll be a hefty bill. Still, someone might install a pump some day, so it's reasonable not to allow it.
BTW, the system is used in France too. I've seen a setup somewhere in Provence, somewhere near Apt. Can't remember the name of the village. Was in a very expensive villa though and it's main purpose was for the garden and the swimming pool. And now I think of it, I've also seen it in Switzerland.
I've been living here for over a year now and haven't found any disadvantages yet. We use about 10.000 liter (2500 gallons) of drinking water per person a year. That was almost double when we were living in Brussels, without a rainwater system. TBH, the water in Brussels came from the Ardennes and was much better to drink and to cook. But that's no longer happening...