The stands look like they will work fine. I would be concerned about one detail that can easily be dealt with. The poppets[swivel pads] appear to be mounted into the vertical standards by drilling a hole in the end of the p.t. 4x4. If this union were to be side-loaded, it could split through the side of the 4x4 due to the allignment with the grain pattern in the wood. I would wrap the end with epoxy and glass tape, hose clamps, or even a whipping with parachute cord to prevent this. The poppets appear to be from Brownell or Hostar stands. Being the most expensive part of commercial stands, you could easily duplicate the lower halves with pipe and angle economically at a local welding shop if you wanted all metal stands. Metal stands are available through West Marine.[I buy them on my Port Supply account regularly.] They can also be ordered from the Hostar website.[they build hydraulic boat trailers similar to Brownell's]
I'm not sure that you're looking at thesame stands that I am, but the main supports that I see are more like 6x6's or even bigger. I don't see them breaking out. Stands are for side support, they keep the boat upright. They really don't hold the weight of the boat.