Welding kayaks isn’t that difficult once you get your eye in. However any boats made from cross linked plastic have additional issues as the the temperature difference between welding and blowing the plastic apart is a very narrow margin.
Quite often even the best welded repair will have some level of weakness and could fail. Some paddlers add wire mesh to strengthen the repair and I myself have found this to be very successful and almost bombproof depending on the location of the weld.
However there is an alternative that I have seen on a number of boats over the last few months. It looks rather industrial but seems to work really well. Below is a series of images from Nick Wright who used this alternative approach to repair a Jackson Kayak play boat.