Not the strongest weapon ever, but it's not that bad imo. Flails are always at least decent damage-wise, and often surpass even further tier weapons when upgraded. They allow anged builds in need of a melee weapon to save some SoUs in exchange for some reliability and melee surprise attacks (which, as others noted, shouldn't really be used much in this case), and with a 16 strength requirement you don't always need to use a potion of mastery in order to equip it earlier.

Outside of this scenario it's definitely a bit lackluster - it may be stronger than most weapons, but melee builds often either prefer hitting more times rather than doing lots of damage in a single sweep, or have a way to amp up their damage without penalties. And, of course, it's basically unusable for rogues.

But all things considered, I think it's fine as of now. Neither bad nor good, an option like many others.