Type balance isn't solely governed by type matchups. Tools that types have access to, like Fire's burning, Dark's variety, and Poison's poisoning, are also factored in. The selling point of Steel is that it resists many types, whereas the selling point for Poison is that it has access to the poison status ailment. In turn, the strengths and weaknesses of the types cannot be used to justify inherent type imbalance in a new mechanic.Some Types can hit more Types for super effective damage(2 for Steel and 1 for Poison). Some Types resist more Types than other Types(11 + an immunity for Steel while 3 for Poison). By that same logic there is no balance in the Types either. Some Types just get more benefit than others they all can't have the same amount of benefits and hindrances.