Good speakers are good speakers; simple as that.
Floorstander are perfectly capable of functioning as rear speakers as long as you can place them properly. But placing rear bookshelf or satellite speakers can be a problem too.
If you have a cramped room, where the floorstanding speaker have to be against the wall beside the sofa, that can create problem. But in a larger room where one section or one end is set aside for listening, there would be room to have the speaker set back from the sofa, and place in the optimal position.
So, it has nothing to do with whether or not the rear speakers are floorstander, and far more to do with whether floorstanders can be placed properly or ideally in your room.
Even in a less than perfect room, reasonable compromises can be made to produce reasonable sound from rear floorstanders. And, in many small rooms, even the placement of bookshelf and satellite can be difficult, so the problem isn't just floorstanders. We have people with problem rooms write in all the time for advice on how to place a 5.1 in a odd room.
The trick is not to think about where the speaker should go, but to concentrate on what you want to accomplish in placing the speakers, and then do the best you can to accomplish that in your room.
Just a thought.
Steve/bluewizard