I think hes a trainer that knows his horses better then the majority here think they do. He also is one of the biggest advocates of safety you'll find.
I believe the stunt itself isnt all that dangerous. if the buckets tip the panel would just fall to the ground and I have a hard time believing he would do something that was to dangerous.

"Blessed are the flexible for they shall not get bent out of shape"
"You can discipline and Discourage or you can Discipline and Encourage"
"Gentle in what you do , But firm in how you do it"