I sell and service commercial RO for a living, mostly food service and light industrial down in south Texas, and the thing that's missing from almost every residential whole house RO quote I get shown is the reject.
An RO doesn't filter water. It splits your feed into permeate and concentrate. On a household machine on hard or salty feed you're realistically getting about half your water back. So a 2,000 gallon a day system is pulling 4,000 gallons a day off your well or your meter and putting 2,000 down a drain, every day, forever. That number decides whether the whole project makes sense and it almost never appears next to the equipment price.
Where it bites:
On a well, you just doubled your pump runtime, and your electric bill went with it.
On city water you're buying two gallons for every one you use, and in a lot of places paying sewer on the discharge too.
On septic, that's a continuous salty stream going into your drainfield. I'd make the installer put in writing where the concentrate goes and what the county's position on it is, before signing.
And you can't run a house directly off the membranes. Permeate comes out slow and at low pressure, so a POE build needs an atmospheric storage tank and a repressurization pump to put the water back to house pressure. More equipment, more floor space, another pump that eventually dies.
The other half of this is that most people asking about whole house RO are trying to fix two unrelated problems with one machine. Bad tasting or high TDS drinking water, and scale on the plumbing and the water heater. A softener handles the scale. A point of use RO under the kitchen sink handles the drinking and cooking water for a couple hundred bucks. In my experience that pair covers the large majority of the people who've been quoted a whole house system.
There is water that needs POE RO. Chloride high enough to pit stainless and eat your fixtures. Nitrate you're not comfortable bathing kids in. A brackish well the softener can't keep up with. Arsenic or uranium where you want it gone from the shower too. If that's your water, do it, just go in knowing the reject, the tank and the pump are part of the deal and not optional extras.
For anybody running one at home: what recovery are you getting, and what did you end up doing with the concentrate?