Yes, a plumber suggesting chemicals is like a doctor suggesting witchcraft.
My experience was when I was living in a rented townhouse by myself. It was Christmas eve and my tub would not drain (only tub/shower in the house). I left a message for the landlord, but then I thought "maybe I can take care of this myself, clever me" so I bailed out the water and poured Liquid Whatzit down the drain. Nothing happened, so like an idiot I poured more in and filled up the tub to have a lot of pressure behind it. I was still waiting for it to work when one of my cats ran upstairs acting all freaked. He led me downstairs to the kitchen which was right below the tub, where there was a steady stream of water coming THROUGH the light fixture (eek!), across the cabinets, down through the cabinets by way of all my food, and onto the counter and then the floor. Of course there was a tubful of water mixed with Liquid Whatzit built up behind it, so I could look forward to a huge mess.
I called my landlady again and managed to get her on the phone. "But it's Christmas Eve," she screamed at me. "I know," I said. "Guess what? It's Christmas Eve for me too, and yet the water is still pouring through the ceiling." (I didn't admit to her that I'd used the chemical, because by now I realized my mistake.) She told me to try to find a plumber and pay for it myself and then take it off the rent, but I refused because that wasn't my job and I didn't have that kind of money up front. It soured the relationship with her permanently, because apparently she was of the firm opinion that emergencies should only happen when it was convenient for her.
So. . .no chemicals in pipes. None. Zip, zilch, nada. Considering how extremely caustic these things are and therefore how dangerous not only to the user but to the ecosystem, I don't feel they should even be sold.