The combos were the first to go into RV's. The early models didn't work that well, but I'm sure that the newer combo models work much better. When they were first offered in coaches years ago, manufacturers were not really thinking about stackable units, consequently there wasn't room for a stacked unit in older models.
I would bet that most people who currently have a combo unit now, would definitely go with a stacked unit in a new coach. Keep in mind that most coaches that have washers and dryers are larger and space isn't that big of an issue versus the value.
I don't even know if there are any manufacturers still offering only a space for a combo unit, which kind of tells you what is preferred. I'm sure someone will find one brand, but not the norm.
Our last coach only had room for a combo and I just wasn't interested. Our new coach came with the stacked washer and dryer that we had already decided was a must have. Typically, we do two loads when we wash. We do the wash and while its drying we can start another wash, cutting our laundry time down.
Don & Mary
2019 Newmar Dutch Star 4018 (Freightliner)
2019 Ford Raptor