Is jailbreak harmful?
Yes, a little but not so much. If it screws up your device you can just restore your device and it will be back to normal with the jailbreak off of it. So really no.
It can make your iPod a little slower sometimes, but no not really
If you don't do everything exactly right. I messed up one little thing. It ruined my iPod and I had to reset it. I lost all of my memory.