I agree with Pinto...not all RG6 is of equal quality. You have to separate out problems caused by poor quality off-shore RG6 from those caused by the use of CCS at the lower frequencies.
In addition, for those sending DC power up the coax, you have to consider whether the additional resistance of steel has an impact on the DC power available at the distant end. That would be easy enough to measure.
What I would like to see is a measurement of matched load line loss of a 1000 ft of CCS RG6 vs 1000 ft of pure copper RG6, of equal quality construction, at 100 kHz, 1 MHz, 10 MHz, and 30 MHz. That would tell me whether this is a real problem or not.
Also, it is becoming harder to find solid copper too. I can’t find ladder line cable anymore that isn’t CCS.
I set up a 1000 foot loop-on-the-ground antenna this Fall, using CAT5 UTP. The cable I bought was Copper Clad Aluminum (CCA). The DC resistance on each conductor was twice what copper wire would have been. I have no idea if this impacted the antenna. I heard plenty of DX on it. Maybe I would have heard more on a pure copper antenna... maybe not.