Durin's Barrage

[Not John Smith]

This seems waaaaaay too powerful.. super high mobi + high damage primary attack + insanely high firerate + undetectable instakill secondary attack that can't be countered by any amount of health + it heals you + you can dash + that dash gives you invincibility + chain attack instant recharge + set bonus that heals you again and deals more unavoidable damage

The only downside being what? It's in your primary slot?