Not a plumber but have done some basic construction work on sites and in houses. A lot of modern plumbing uses PVC pipe or some plastic equivalent. Pouring boiling water into a drain that's clogged shut can trap that water and transfer the heat to the pipe leading it to deform or the joints to come loose.

What I would personally be more worried about is if it's a slower leak. Your drainage pipes have lots of fat and such in them. The hot water will melt or dislodge chunks of it that will reform into a larger problem farther down the pipe.

Sodium Hydroxide gives off a lot of heat when dissolved in water. By giving it a head start with pre-boiled water you risk it splattering and that stuff does not like human skin, eyes and mucous membranes.