Not a doc, but here's what I have been able to gather from looking at the literature I could get my hands on:
Soapsuds enemas are generally more effective than other large volume types, both based on qualitative reports as well as quantified measurements of returns.
Most serious reports of injuries from enemas are connected with stupid behavior that leads to physical trauma or use of dangerous solutions (too hot or chemicals that cause colitis) very strong soapsuds may be an issue in some susceptible people, but in most of the case studies that I found it was not known what specific additives were used so it could just be detergents, excess lye, or other additives.
However, soapsuds enemas are known to cause inflammation of the bowel mucosa. This is not unique, as phosphate enemas, glycerin, and even tap water show some inflammation.
It does not seem to be known what the long term impact is. It is known in other situations that chronic inflammation can lead to issues, but I haven't found any studies of what bowel inflammation does.
Doctors who err on the side of caution may advise against it based on the fact that we don't seem to know what the long term effect is, and there are other ways to handle constipation that may not have the unknown risks.