I cannot vouch for how well these work as fishing rod holders, but they work very well as part of my outrigger stabilizer pontoon system (Google it) on my canoe. They're very sturdy with a variety of mounting options. Just looking at the design of these, I think there are probably better rod holders to actually use for that purpose. There isn't any notches or openings to help secure your rod in the holder (like around the reel), so I would imagine it would be easy to lose a rod in the drink with these.
For my purposes, I used them to hold a three-foot length 1.25" diameter PVC pipe, so I simply drilled a hole through the holder (and pipe), and installed a quick-release pin so the pipe (which is then attached to pontoons) stays in place.
Excellent product for my purposes, but you may find differently for use as a rod holder.