Caustic soda based products work by generating heat and dissolving organic matter. They can shift a soft hair-and-soap plug in a basin, but they do almost nothing to a compacted fat deposit, wet wipes, scale or roots — and they are useless in an underground drain where the product simply sits in standing water and gets diluted.
Worse, caustic products damage old lead, copper and some plastic waste pipes, and they make the job hazardous for anyone who then has to open the trap. If a chemical has been poured in and hasn't worked, always tell the engineer before they start.
Professional jetting is what actually clears a serious blockage: water at high pressure, delivered through a nozzle chosen for the blockage type, cleaning the pipe wall back to bare pipe rather than punching a hole through the middle of the obstruction.