Its what you feed it!
What usually deems a disposal useless, is that it's dull. Waste King used to put in its manuals, that ice cubes, egg shells, and chicken bones are good for disposers.When we moved into our current house, the old Kenmore was dull as can be.
I started throwing those items down it's throat, along with a few pieces of small broken glass, and now it takes anything you throw at it. Glass is what plumbers use to resharpen the blades. No thick pieces, just like the sides of a regular drinking glass will do, and not too many. Again, ours works like it's new! Use your discretion.