Is 6000 MHz memory clock too risky for a mobile GPU?

I have never seen a 6000 MHz memory clocked mobile GPU by default. Why is that? Even the latest high end GTX 980M has 5000 MHz memory clock. Many high end desktop GPUs out there have clocked at 6000 MHz and some even at 7000 MHz.

I am currently using my overclocked GTX 870M at 6000 MHz. It surely adds considerable heat to the GPU but the gains from it also good.

I have been using GTX 870M like this for six months and now there are some crackling noises come from GPU when it is stressed....

Is 6000 MHz memory clock too risky for a mobile GPU?