To answer the question, "What can go wrong?":
1. You can use water that is too hot, too much soap or other additives and boil/pickle your own insides.
2. You can add too much alcohol and kill yourself. Alcohol is NOT FOR BEGINNERS.
3. Plain water can be dangerous too. It is readily absorbed into the body through the intestines. Absorbing a lot of water this way can crash your levels of minerals such as potassium (vital for cardiac functions).
5. You can insert the nozzle or tube carelessly or without lubricating first and irritate yourself this way. If you are super aggro with insertion you could poke a hole in your intestine. This last is deadly.
6. You can overpressurize your intestines and blow them out like an old tire. This is highly likely to kill you.
7. After taking the enema you can get careless about what is going to happen when you have to expel and make a filthy mess.
8. You can also do stuff like share enema equipment or use it when it is not clean. This can mean something extra in the enema - bacteria etc.
All of the above are easy to prevent but they are all potential hazards. It's good to know the risks you are running before you get into something.