Compression fittings should not be used on brake lines. I agree.

But here's the reality: I have seen plenty of them used on brake lines for the better part of 45 years and don't remember seeing a single failure due to the fitting itself. Rusty or scaly lines where they cannot grip properly is a different matter. But I have not personally seen a fitting connection fail that wasn't compromised in some other way.

I have however, seen many double flare connections fail. Usually due to poor workmanship. They take practice and a good tool. Often both can be lacking.

Not an endorsement for using compression fittings on brake lines, just my experience.