I've seen up close and personal at Thanksgiving time what a lot of celery strings (strands form unstringed celery) can do to a disposal. This was a feast for 20 and my friend stringed 6-8 stalks of celery. The disposal clogged up tighter than a drum. A plumber had to come and take apart the trap. Needless they got paid emergency holiday O/T pay for the visit.
Some exceptions exist to what you grind up. When you have a disposal that is very powerful there are exceptions. So my experience says don't grind up the following:
1. celery (unless it's stringed first).
2. egg shells (not sure why but remains seems to sick around too long)
3. Bananas woody stem ends. The peel seems to grind up quite nicely, but cut off the stem ends and throw in trash.
4. Bones...typically beef. These are too dense. I've ground up chicken bones but many sometimes disposals get jammed as the remains can wedge between the blades as they splinter.
5. Avocado skins seem to get wedged as they are hard and slippery and get between the blades.
6. no grease, at all.
6. no woody plant or flower stems ...asparagus.
You can sharpen disposal blades by dropping cubed ice down the disposal and grinding up.