There is already a restriction.
That is its price.
Why would you buystock 100 miniguns, when no-one is gonna buy them

The biz will automatically go in loss, the owner wont make the same mistake again.
Also, Minigun is VERY expensive.
Though its ammo costs 5$ per bullet (which is less than sniper).
But consider its fire-rate.
You are actually spending 500$ if you fire minigun for 10 seconds.
Actually M4, Ruger and M60 are more powerful than minigun.
Since you cant crouch/jump while holding a minigun. There are high chances of dying if you miss the first 5 shots.
Rocket launcher is not overpowered at all.
To efficiently use it, you need to have good amount of ammo(expensive), knowledge of rocket-slide glitch (which is risky as hell, not allowed either).