I think the answer to this is: "we don't know." We do all sorts of things to our bodies that they are not adapted to (a lot of processed stuff that passes for 'food' comes to mind). Our bodies are amazingly resilient and have systems that work to maintain homeostasis (balance). Basing decisions on what feels good isn't necessarily good for the body. For instance, we eat all sorts of "comfort" foods that give us pleasure at the time we eat it and disease in the long run. Fucking feels great, but one can also get lots of diseases from that... we could make a long list. Lots of human activity involves risk/benefit.
Putting fluids into our colons rectally bypasses the 'natural' process of digestion. Our bodies have ~24-27 feet of intestine and our colons amount about ~3-5 feet of that length, the rest is the small intestine. Medical science knows lots of really cool stuff, but the body is an infinite universe and each time we make a new discovery, it reminds us how little we know in the grand scheme of things. For instance, the latest evidence informs us that we have about 3x more bacteria living in our colon than cells that make up our entire body (we live in symbiosis with bacteria and based on that understanding, one could ask: "what am i doing to my gut flora with this particular enema?"). To demonstrate how knowledge changes, a few years ago we thought the ratio was anywhere from 10-20x. I see science as more of a process of continuous learning than a system that produces static absolutes. If one is basing decisions on scientifically derived evidence, it's really (hopefully) "current" evidence, and that decision may be modified as more evidence becomes available.
Making health decisions based on scientific evidence makes the most sense to me, but i understand evidence is not always readily available, it can change, it is subject to confounders, bias... i.e., it isn't perfect and it takes some understanding to spot flaws and know how to use it.
A common assertion in studies is: "more studies are needed...." to address the question being asked.