Old clay liners are where you have to watch it. I've swept chimneys for 11 years and learned the hard way on a 1920s house over on Chestnut Street, went at a spalled liner with too much force and knocked loose chunks right into the smoke shelf. Real soot comes off with angle and speed, not brute force, so keep your rods spinning and let the brush do the cutting instead of jamming it straight down the wall. A stiff wire brush is fine on firebrick and newer liners, but on old clay tile you want poly or a softer wire and lighter passes, more of them, and you'll get the same result without cracking anything. Do the fingernail test on a test spot first: if the glaze is already flaking or the tile sounds hollow when you tap it, back way off, because once you gouge a liner it's a repoint or a reline and that's a whole different job. The lady wasn't wrong about technique, she was just used to her fireplaces and not the soft stuff underneath.