Traditionally they've always been unreasonably expensive (relative to individual units) but in the last few years they have come down a lot and now they're only mildly more expensive than individual units.

I suspect once two units or a unified unit hit the same dollar price you'll see a lot of people switch over the unified systems just so laundry is more "set and forget." Plus some of them have smartphone notifications now when the laundry is done (so no listening for the alarm, etc).