Secondary adamatium: Savage hulk gets angry enough to take a few, WWH takes all.
Primary adamantium: Colossus will be almost unbeatable except for BFR. He will be about like Juggernaut.
I assume Colossus will gain proportional strength with durability. I don't see why he would not. His muscles are metal. He sure gets stronger going from flesh to osmium doesn't he?
It has been on rare occasions that bricks have dented primary adamantium. That would be like making a small intention on your skin at best for Colossus, or maybe like a small scratch at best. He could just sit there and get pounded on and not worry about anything. He would be as boring to watch as Juggernaut with all sorts of PIS used to stop him.