Eric Cortina used CLR regularly and showed a LARGE empty container that he finished up with on one of his videos. He said that even though he never had any problems with any barrels that he cleaned with CLR, he stopped using it because it voided the warrantee from the barrel manufacturer.
I submerged a SS brake in CLR. It was my intention to leave it in for an hour or so, but I totally forgot about it. The next day, I remembered about the brake and removed it from the CLR. All the built-up carbon just floated off the brake. The brake looked slightly different in the finish but it was fine. I've repeated this process without issue.
I think CLR is reasonably safe for use and would definitely use it to attack carbon that is resistant to removal by other methods.