I dont understand what makes them unsafe
Coffee enemas (as with most anything that's considered in anyway "alternative") will almost always be labeled by conventional medicine as "dangerous", or "unproven", or "unscientific", or in some other way that's meant to convey a disparaging assessment. There's so much I could write about my own (positive) experiences, but for now, will leave you with a couple of links to YouTube videos that summarize a lot of my own thinking.
The 3 Best Natural Ways to Stimulate your Vagus Nerve to Improve Digestion
The Miracle Probiotic That Cured My Chronic Constipation | E Coli Nissle 1917 (part 1)
E Coli Nissle 1917 | What makes this miracle probiotic so special? (Part 2)