Billy the Blitz... isn't underrated, he's legitimately the worst 5-star yellow bird in the game (or tied with Cyril for that distinction, depending on if there are fans present or not). It's just that even the worst yellow birds aren't as ill-suited to PvP as certain entire colors are (yellow and white are unambiguously the strongest PvP colors). Don't believe me? I happen to have in my collection Billy the Blitz and El Jefe, both at P5 with their super shots at rank 5, so the comparison to be made is as fair as possible: Billy does 2,296 damage with a super shot timer of 3. El Jefe, who is also not especially well regarded, does 2,446 damage with a super shot timer of 2. Typically the longer super shot timer is the trade-off that comes with extra damage, but Billy does less than the weakest 5-star yellow bird with a 2SS timer (my Lady Bacon is also P5 but has her super shot at rank 6 so the comparison isn't entirely fair, but for reference it's 3,572). His attack adding additional combo hits is nice in theory, but at 3SS most matches are already over by the time he would be ready, so what you really want from a 3SS bird is a finisher, not a low damage attack that boosts the next one. Bad for a yellow bird is still an entirely different metric than red or blue though.