From fairly recent experience airport assistance with a wheel chair on arrival in Bangkok has been excellent.
However on departure unless you have you have your own chair extra time will be required because the service only kicks in after booking in. From the drop off point to check in is about 100 metres and seats are only nearby the counters. If this distance is too much it will be necessary to contact the airline desk so you can be picked up from outside. Now if you've got to check in Premium Economy will be an advantage because the zig-zag line will be shorter. Once booked in everything becomes very good with two exceptions. Firstly you may have to wait on the nearby seats for 15 to 30 minutes for a chair and a pusher, hence the need for extra time. Secondly the speed at which the pushers go down numerous ramps is quite scary! (This seems to be an experience with most airports.)