Correct, dehydration-related complications are possible during opioid withdrawal, and I agree that it's hard to put a number on cases. The other significant risk is suicidality, which is very common during opioid withdrawal. In general, I tend not to recommend full, rapid, medically-supervised (or unsupervised) withdrawal from opioids, as the best option for most people is a transition to opioid agonist therapy with buprenorphine or methadone--which have profound mortality benefits. There are always exceptions--especially in people who prefer antagonist therapy with long-acting injectable naltrexone--but agonist therapy has the most data.