There are two big concerns I have:
1. Hygiene and cleanliness. The disposable specula and hoses are a big help but some places still use a metal speculum (which I personally adore) but these need to be carefully sterilized. The colonic machine should be sanitized between each use by using a disinfectant and the work surfaces and treatment surfaces (pillows, treatment table) should have clean linens etc. The therapist should understand glove changes and another chance for contamination is the lubricant. I prefer to see the lubricant squeezed out on a paper towel then transferred to wherever. I went to one place that was using a large jar of Vaseline, and the therapist was dipping her finger into it. I stopped her and explained that she was possibly contaminating the jar with her gloved finger, as she had touched the equipment and work surfaces before putting her finger in the "community" jar.
2. Poorly Trained Therapists. Training of therapists is spotty at best and is often a sort of apprenticeship program. I had a very uncomfortable insertion by a young girl who obviously didn't have much experience or understanding of what she was doing. I asked her how many colonics SHE had had, and she admitted that she had never had one. I was floored. How could she understand what she was doing to someone else?
I am not promoting this organization, but at at least I-ACT (International Association of Colon Therapist) has an anatomy education component, a requirement for training, testing, etc.
Most therapists are kind, gentle, very skilled, and safe. But there are production line operations that take your money and treat you as if you were a car in for an oil change. Here in Southern California there are some high volume chain type organizations that offer colon therapy, and while they may be clean, they offer less than a satisfying experience.
That being said, there is nothing more I love than spending time with an attentive therapist having my bowels purged and my colon massaged. I have a favorite place I go when I am in San Diego. For an hour, I am alone without my cellphone and my stresses melt away. Not only does she give a nice thorough, aggressive colonic, she also works your neck, and feet and is skilled at lymph drainage massage which is quite pleasurable.