IIRC, Leviathan can't play Sentinels, and he's also not good at Controller. In a Thai interview, crws mentioned that Leviathan has to wait for Crws to make the call on when and where to smoke. This unnecessarily increases Crws's workload as an IGL and sorta makes FS 1 step slower than their enemy. Most of the time when Leviathan decides to smoke on his own, the timing or placement isn't good. While Leviathan's aim is solid, his game sense is lacking. That's why FS swapped him for Seph1roth, who has slightly worse aim but is smarter and more flexible overall.