Devil's advocate: the back of those drain cleaner solutions make a point that the solution should never sit or pool in any surface or pipe. You have to run water and chase it down so it doesn't corrode the pipes. I know where I live some of the pipes have "flow issues" because of bad design, and if there was already a partial blockage I can see drain cleaner corroding the pipes...

We just mechanically clean them out ourselves, anyway.

Maybe if you have metal drain pipes. They sell the stuff in plastic bottles. If you have PVC drain pipes, it isn't going to disolve them.

If you ask them why they say its because it'll often dissolve the pipes before the blockage. Most of the trades people I talk to aren't looking to maximize their work. I've had them turn down jobs for being too gross and I don't blame them.

They'll generally use a snake/auger. Home Depot sells a basic one for $15. The beefier, further reaching ones cost more. Or, for smaller jobs, there are single-use plastic snakes.