I used to use mine almost everyday , but I've moved now so don't have one.
Mine was a three sectional wall bag it was filled with mung beans if I remember correctly.
If you are doing very high repetitions you may start to take the skin off the knuckles ,even if you have good technique as the center starts to become more compacted from the repeated impact.
During the training , stop from time to time and hit the wall bag near both the edges to force the filler back into the middle.
You could end up hurting yourself if you do a lot of high volume work on it , without being instructed in its use by a qualified Sifu.
Definitely do not do elbow strikes on it , the shock of the impact will go straight up to your brain.
Punches , palm strikes , fak sau are ok though.