Well praiseluka gave you the explanation on what is a cancer team. It's either unkillable due to triple healing/barrier+debuff or it is a team that 50% of the player base use since it is strong and easily accessible.

Bottom line however is to not care about wether your team is cancerous or not and play the game using the lineup you want. If it's annoying to others but you enjoy it then s*cks to be them. Don't let other players opinions or remarks compromise your experience of the game.