I have a k pouch and not a j pouch (which is what they are giving you)...yours uses the natural biological exit (the anus), mine does not...in the k pouch they have to build a one-way continent valve out of your own intestines to prevent things from pouring out...and that valve can be delicate if not well treated or for a number of other reasons...my pouch (the original one that is 33yrs old) is doing just fine (knock wood!)...it is the valve that has had its share of problems and therefore requires some work...(plus I have D'Heler's-Danlos syndrom which means that my body does not heal very well and often requires redos)...also the k pouch is attached to the abdominal wall and I am hernia prone so you can see where that would cause problems..
For the rest, quite honestly, I would not worry too much...a good doctor/surgeon always warns you of the potential risks of surgery and the possible outcomes that are inherent with our procedures...talk to him/her if you are concerned or having doubts.
Hugs and good luck
ps...Stress is very, very normal before these surgeries