There are reasons not to use a FWS - mostly it has to do with structural integrity. The companies that use them have problems with them (probably Newmar's number one slide problem comes from FWS units. Newmar has changed their instructions from jacks down and level to slide out first and then level because of structural flexing.)
Tiffin will not build FWS units because they can't guaranty the wall structure so they install 4 slides. My front slides are 3ft deep and 16 ft long in my 36 ft motorhome and add much more square footage than a FWS. These huge slides are moved by HWH hydraulics which are more than up to the task.