Can iRay or Mental Ray, render every scene like Vray?

Can anybody tell me whether iRay or Mental Ray can render every scene like Vray?
Mental ray is an advanced render. Why do people need to go to 3rd party?

V-Ray is just simpler. Mental Ray is more complicated but have better effect in my opinion. If you know mr well then it renders faster more complicated scenes. And iRay as you probabely know renders extremely fast.

I'll keep using Mental. Thanks for ur response.