We know that Ballista's powers don't work on doors, but (this is just a guess) some people may think that the door was so invincible because all of Ballista's attacks were on a door that said "Pull", so he was pushing on the door with his powers. However, this can't be the case as in the last part of "stickman vs. door", we can see that you have to PUSH the door open. (I'm talking about the part where - spoiler alert - Ballista gets chased back out by a dog)