FS is not overrated, neither is CoC. Luffy being shit in using FS or AdCoC doesn't mean they are overrated. Both are only handful and mighty people can use it.
No.
Let me address FS first. We literally have people claiming that other Yonko Commanders like King or Marco won't be able to touch Katakuri due to his FS even to this date lol. When it has been proven wrong, multiple times. Speed counters FS. There is no use for FS if you cannot react to the attack in time, as shown by Kaido's TB on Luffy on the rooftop and his Boro Breath. That is the overwank that I'm talking about.
And let's not act like people weren't wanking Adv CoC and downplaying admirals for a good while until Oda showed us that Snakeman move and Yamato's attacks which barely damaged Kaido. We have been told that Garp almost killed Roger and vice versa multiple times and Garp hasn't been confirmed as a CoC user yet.
Thinking that a character is far and beyond another character due to possessing a single ability is wild and it is not something a good or balanced powersystem should have. But unfornately the boys in the Battledome use these simplistic and straight up wrong logic to push their Katakuri or Yonko agendas.