What kind of complications might happen from a webbed finger or toe repair?

Flap loss. Perhaps the biggest risk to repairing (or desyndactyly) webbed toes is the risk for flap complications. This could be minor such as separation of the margins to full thickness loss of the flap. In rarer cases, loss of the digit(s) could occur in the face of significant infection of the site. The procedure is straightforward, but consider all surgeries carefully, weighing all risks and benefits.
Loss of skin graft. Additional skin is needed, supplied by a skin graft. Incomplete take of the graft or impaired blood supply to flaps can occur, but not common. Late creep of the newly created web space can occur, requiring a minor revision.
"Web Creep" Aside from the usual risks of surgery, sometimes the skin may not grow as fast as the bones (if done in child). This can lead to an appearance of recurrence.