New types are unnecessary and it's very easy to get them wrong.
This statement makes no sense. How is it "easy" to make them wrong? How often has it happened in the past? Are you suggesting that steel and dark types were "wrong"? I'm pretty sure they're fairly popular.
Dark, probably not. Steel, yes. Steel wasn't a problem in the past because, at the time, Steel Pokémon and moves were very scarce in number to make a huge impact. But look at the metagame now: Steel Pokémon are EVERYWHERE! They make a powerful combo with Dragon Pokémon because of how well they cover each other's weaknesses. Also, was it really necessary to make Ice weak against Steel, when it was already weak against three of the most common offensive types in the game?