Are stoma reversals always successful?

Not always: The success of a stoma reversal depends on many factors including the original reason for the stoma, the overall condition of the patient and the amount of bowel remaining. Often a temporary stoma is constructed for diverticulitis. In this situation, reversal is often successful assuming a healthy remaining rectum. Each case should be looked at individually for the pros and cons of reversal.

Sure! Most times!: Every operation can have complications, even the "easy ones!" stoma reversal is one of the straightforward operations general and colorectal surgeons do routinely, with very high success rates, and low complication rates. Talk to your surgeon and ask about which and how often risks/complications are seen, but don't obsess! of course, your general health plays a role here also.