Actually, it may be the second worst suit Tony made. Why? Not exactly that its concept was bad, it's that the suit was a prototype. As such, many things tend to malfunction like when Tony tried to fire a missile at one of the helicopters that were attacking his mansion; it failed to work so Tony had to manually pull the missile out, and had to shoot a repulsor to detonate it. So why 'second' worst? That's because the worst is Mark II, which does not have a single weapon EXCEPT for the repulsors. Imagine Tony used that to fight Thanos. He would be turned into mash potatoes for Thanos in 10 seconds!