The big advantage for me with FTL over focused leader was freeing up focus which has been priceless in the Arena, honestly focus alone probably made winning 10x easier as it allows you to setup completely unexpected "one shots" on any target before they can react at all to bubble, shell, iceblock, etc

Aside from that ive also found FTL to be a bit cleaner when switching mains as it just always works flawlessly. With a focus based leader setup any alts whom are currently under certain CC effects like gouge will be unable to acquire the new leader as a target to focus until the effect expires