what i can tell you all is this- mash hop and FWH seem to keep flavor and aroma in my lagers AND reduce the bitterness.
i made an IPL with a ton of citra, and while its high AA hop the beer was noticeably less bitter than you would expect based on IBUs/hop amounts. still plenty of flavor, just not as bitter as you'd expect from an India. in my opinion- just the right amount of bitter.
now- i have very clean, very low alkalinity but high pH water, use a little gypsum to get pH down to low 5s, and i brew in a bag with pellet hops. most of the mash hops stay in the bag, but not all. sometimes i FWH in the bag, sometimes i've just dumped into the wort as i didnt do a sparge at all.
i really do think that given an appropriate style of beer, mash hop and FWH will lower the percieved bitterness while keeping a good amount of flavor and aroma. my lager is hoppy in flavor but just a little touch of a smooth bitter to cleanse the palate. and the IPL was full of hop flavor and aroma, but no harsh bitterness that overpowers the lager. ( this is my pet peeve with most commercial IPLs i've seen, way too much bitterness. a thin wort just cant stand up to it. mash hop and FWH have made it easy to create an IPL that actually has hop-to-wort balance.)
if you're a crazy hophead and love the bitter- well obviously this is gonna be a waste of hops to you. the only reason i could see a hophead having a reason to try mash/FWH is if you wanted to layer half a dozen different hops in one beer. you could get smooth aromas/flavors from some mash/FWH varieties, bitterness from others at 90, another and 60, and then the typical volatile aroma/flavor from normal late additions, dry hops with another variety, etc.
but it really would be a good idea for anyone who does not have low alkalinity water to do some real research and see if this is a deciding factor in whether or not mash/FWH will work for them. the fact that it works so well in lagers and other lighter body beers leads me to think that it may be a deciding factor- all your traditional lager producing regions have relatively low alkalinity water. germany, the czech, austria, etc.