I will agree yes and a no. Tank is definitely the one most important position. However, Onatel is closer to an old school tank with an upgraded ability as compared to a destroyer class like GM. Onatel may not kill you but it delays the opponent heavily. So in raid defense, hero synergy is more important than just a super single hero. Onatel is one where she delays the opponent enough for her allies to fire and deal some annoying damage herself. She is also tanky enough to hold her ground on her own.
Most here scrutinize the individual hero as if the raid defense is a single hero defense. The team consists of 5 heroes. So it’s best to study the best combination you can find with your available roster.