I just returned from both cities and was very frustrated at the lack of access. I am ambulatory, but cannot walk long distances, can only go up a few steps with assistance or a handrail to hold on to and cannot climb ramps or stairs. I was unable to enter the Temple of the Recling Buddha in Bangkok because it had 4 high steps and no handrailing and I was alone without anyone to help me. I didn't even try the Temple of the Emerald Buddha or the Grand Palace because of that.
In Beijing I walked up a steep ramp to the landing, but was so out of breath when I got up to the top, I had to sit and rest for a half hour to get my breath. I had a young man for a guide that I hired outside the Forbidden City and he helped me up the ramp and then took me down the closest ramp to exit. Since it was clear that I was not going to be able to actually go into any of the areas, I then left. And, again, based on that experience, I didn't try the other sights such as the Temple of Heaven, Summer Palace, etc.
Based on my experience, I would suggest that anyone who has problems in this area please check out the issue of accessibility before planning a trip to avoid such frustrations. It is very aggravating to go so far and then not get to see the major sights due to health issues.