Not a doctor and would love to hear feedback from the true medical types on this, especially on epithelium regeneration and risks from epithelium damage.
Can they? Yes. If you want to make sure you never have a health problem from an enema then don’t take one. Is it an unreasonable risk? You need to make that call based on what you do and why.
Most serious issues from enemas can be minimized by being smart. The people who make case studies and the news generally poison themselves or damage their colons with poor solution choices or cause physical trauma with poor equipment use or choices. Make sure your water isn’t too hot, be careful what you put in it, and watch how you use your nozzle and your risk of damage drops significantly.
Are enemas benign? I haven’t been able to find anything saying straight up yes or no. Based on your body and how you use the enemas you can change your bowel’s habits and have “rebound constipation” for a while if you stop using enemas. I haven’t noticed this, but my bowel is “pre-messed-up.” It seems like it might not be as big of an issue with mild enemas as it is with stimulant laxative use, but I haven’t seen anything definite, nor anything that separates out Fleet-type enemas from mild enemas.
Another potential issue or potential non-issue is epithelial cell damage in the colon. Studies have shown changes in the colon epithelium from soap (6 gm castile soap per liter), phosphate (Fleet), glycerine (Japanese packaged enemas), bisacodyl and tap water enemas. There were multiple different studies, so it’s difficult to rank the solutions. Isotonic saline and PEG-ES enemas caused little or no mucosal changes. I don’t have the medical background to understand the significance of the mucosal damage/changes or if there is one – colonic epithelium regenerates pretty fast, and even under normal situations it is replaced often.
The benefits of enemas is debatable. It is known that they are effective for emptying the bowel and relieving constipation. It is known that autointoxication and cleansing to promote help are pretty much bunk, and I haven't found any scientific studies supporting the idea of the enema to treat colds. Personally I have IBS-C, and in my opinion my quality of life improved when I brought enemas into my treatment protocol, but I haven’t read anything authoritative recommending enemas for IBS, so YMMV. The studies promoting enemas for treating things are generally of suspect quality – not necessarily fraudulent, just not high quality with good sample sizes and excellent methodology.
If you use enemas for sexual fun I suppose you can bring in the studies showing that frequent orgasms promote health 😃 .
Historically enemas have been used for a while and nothing negative has been found in general, more with specific instances, such as with solutions (e.g. tannic acid, turpentine). Be smart and safe and you probably won’t kill yourself or cause significant long-term damage. As has been noted there are plenty of other activities that are likely to be equally risky or riskier.