Sodium Hydroxide is a component of many products such as draino, etc. It dissolves organics, which is what most drain clogs are: hair, grease, etc. It is also what was historically known as lye. So, it would depend on the concentration. I used to work w/ it a lot in a lab setting and whenever i'd get a solution of it mixed in water on my hands, I'd end up with a nice hard water-type slick feeling which was actually the outermost layer of skin cells turning into mushy goo.
As for me, I've had personal experience with Berryman Chemtool and never had any trouble. My last saab ran 191,000 until I was in a wreck. I never changed a head gasket, timing chain, blew a turbo, lost compression, had oil consumption of any kind... However, based on the fact that autorx has such miraculous claims, I chose to give it a try in my newer saab which doesn't seem to have been cared for as meticulously by it's previous owner. If it works, you can bet I'll be it's biggest promoter.