A potato would just make a mess. Something that will shatter and splinter will get into all the small spaces (and fly up and hit the top as well) and clean the disposal. About all it’s really going to due for glass is mask the sound of the glass getting broken up.
If you ever want to try it. Jam as much ice as you can into the disposal, run some cold water and turn it on. It’ll make a horrendous sound and the amount of disgusting muck that’ll it’ll break up could turn your stomach. In my case, when I do that, some of that gunk will come up into the other sink for a few seconds. It’s gross and smells just as bad.
For me, a potato is a bad choice because most of the times that I’ve had garbage disposal problems (i.e., binding up and/or clogging the sink) potatoes have been involved. Usually large quantities of potato peelings, but also when discarding mashed potatoes.
And ice cubes eventually melt, if the disposal gets indigestion from them.