Mulch is fine but will want to settle out in the hose. The more moist it is the more this is true. Dirt can be trouble, especially if it's moist. This is more for folks who use these in conjunction with large stump grinders on occasion. After prolonged use it will begin to build up in the housing like adobe. Eventually it will begin to meet reistance as it comes into contact with the impeller, slowing it. If it's not cleared the fatigue will cause the impeller steel to crack, again after prolonged use.
Short term though, things to avoid? Blocks of wood (scraps from 2"x4", 4"x4", ect.), shoes, carpet, packing straps, and beach towels. All os those things are light enough to pick up and can bind between the housing and impeller and cause failure of multiple components. Grit and gravel will go through but will sand blast the housing and shorten the life of the liner and discharge. Rocks that could do any serious damage are too heavy to be picked up.
Drew Coates, AKA Mr. Force