Concerning 6v6, keep in mind that it is a very hybrid place where the really good technical PvPers are underrepresented because they lack the gear and there’s a big influx of top gear PvEers that can make the most silly beginner mistakes. Add to that the tactical component of braziers and horns and the feeling of class balance can be extremely different between individual people.
As a 1700 gold 6v6 KFM player, I would say KFM are a good middle field class for 6v6 that has strength and weaknesses in any setting, and I personally see not why on earth there are so few of them in gold rank. Maybe because the KFM player base is smaller as a whole.
Vs destroyers, I feel very confident as a KFM since they go red instead of blue in 6v6. My guess is that people perceive the destroyer as overpowered because an AoE damaging and CCing source is a major danger area that suddenly pops up in a confined area. If you have just two stupid people that do not know how to get out of the way of the spin, you have to act defensively for endless (10+) seconds, instead of getting the chance to switch over into the offense. So my guess is since you require team intelligence to counter red spin destroyers, they are stronger with it in 6v6 than normally.
This is also not unique, stealth camping sins for example are often countered better by one good player than by 3 mediocre ones.
Personally, I think destroyer and BD grab restraint is 3 times more dangerous than red spin in 6v6, and I am glad KFM are one of the few classes that can avoid these for the most time.