The amount of fluid a person can take varies with the person, how empty they are, how stretched they are, their position and stress levels. Be wary of units as some units of the same name (pint, gallon) differ in size between the UK and the USA. Long term problems may be caused by distension of the bowels (source?)
1. The colon has no vomiting reflex.
2. The colon absorbs alcohol more quickly than the stomach, leading to a higher peak concentration in the blood.
3. High alcohol concentrations can cause cell damage.
All the normal caveats about mixing mind-altering substances with activities requiring dexterity, judgement or clear communication also apply.
Rule of thumb: dilute down to the concentration of beer, and don't use more than a pint.
Some sources (?) mention a potential dependency problem and loss of ability to have normal bowel movements if enemas are used too frequently.
Some of the more extreme recipes designed to produce cramps will damage the body. If you wouldn't put a liquid in your mouth, don't put it up your butt.
Very hot or cold fluid may damage the delicate colon lining.