I don't know how many times you have done this type of split before but the problems I see are.
>>>> I accept there are risks - but a lot of these concerns are true of other AS methods too.
You are shaking frames with queen cells on them and risking damage to them.
>>>>yes, but the queen cell frame/s are shaken gently, just removing enough bees to see that the queen is _not_ on the frame
Shaking the majority of the bees from the frames placing them into a intermediate hive you risk chilling the brood.
>>>> yes, but is this risk any more likely than having to go through a hive several times to find the queen? It does not take long to do the whole process, and the original hive is closed up again quite quickly.
The queen could be injured, or in the confusion killed by the bees (remember your tearing their home apart this will be panic for the bees)
>>>>> Any intervention carries this risk. All I can say is that the bees are not showing any outward sign of high panic or stress. I work bare-handed, and did not receive a single sting, or any pinging from bees.
The queen could take to the air in the confusion disappear over the fence and be lost.
>>>> the bees seem to congregate on the sheet before deciding to move up into the dark - there are very few bees in the air. The queen flying away is a risk I take by choosing not to clip her.
The majority of the flying bees will return to the original hive.
>>>> We sat and watched them - nasinoving from under the board started very quickly, and very few bees took off and flew back to the original hive.
The majority of the bees now clustering under the board are lost nurse bees who have never been out of the hive some will be very young bees. They are clustering because they don't know how to get back to the hive.
>>>> I have been told that the bees that most easily 'fall' off the frames when they are shaken are the flying bees, so providing I haven't shaken too hard, it will be the nurse bees left on the frames.
I could think of a few more but they are my main concerns.
) and the success rate with it has not been so high. With letting them form their own cluster ouside the hive, and leaving them there until we see the scouts orientate on the cluster, and move off, the bees seem to be convinced they have swarmed , and we don't need to add brood, or put in a queen excluder for them to accept their new hive. If your concern is the number of airborne bees flying around looking for home - our experience has been that there are far fewer airborne returning bees than doing more traditional AS methods, and they settle again far quicker. I am interested to know what potential problems you believe will be caused by this method over and above the problems of any other AS method or natural swarm. 
