If anything, taking over the body from simple astral projection when most simply dont go past Elhki or Vespin, is near impossible. As for getting stuck in different realms and or dimensions, I'd highly recommend using your own Mental Body to guide you back. People who've been Astral Projecting for years document what they see inside of their Mental Body's consciousness, not the subconscious.
There are still some dangers, although not legitimate. Anything with the spirit world is at its own risk. There is nothing that is entirely safe.
As for shields, there is more than one that you can place. I often create reserves within my metaphysical body and attach them to my chakras to be able to fuel the shields and to make sure that there is no gap within them, as the majority of people who attempt to make a shield often think it's "good enough" and don't check for the gaps or holes that can mess with their own energies.
My personal favorite is a double shield. Theres a shield placed within and enlaced in your metaphysical energies, and the shield on the outside on the astral field neutralizes any sort of energy that may come through.