The skills to be a better All- Star player are easier to acquire (basically they require some math and probability solving).
The skills to be a better Piranas player are a lot harder to acquire (basically being able to accurately read your opponents' pill strategy).
An unskilled Piranas player will get a lot of easy wins through sheer luck... with pill manipulation, if you win without overpilling, you most likely have the match in hand. Pill manipulation is also a huge threat which causes other players to overpill, so you also get free wins off your opponents' play mistakes. But against better players you wont get those free wins.
All- Stars is the premier combat bonus clan, brutalizes GHEIST, and has basically only one uphill matchup (being Piranas). Play five All-Stars minimum because their characters need their bonus, and either play mono or find a clan with 3 characters that don't need their bonus so much.