Single flopper shafts work really well for almost any fish ... so long they are tuned properly. Most shafts come from factory without a tuned flopper and thus single flopper shafts get a bad wrap for not holding fish properly ... and many fish are lost in the micro second where you slacken the pressure on the fish a little ... then the flopper closes and fish comes off. Also the slacking and closing of the flopper tends to tear flesh. With a tuned flopper the flopper acts more like a solid hook and much less chance of a fish stuggle making the hole bigger and allowing soft fleshed fish to escape.
I made a 3D print of a tool I use which tunes a flopper in seconds ... and can be used even on a rocky boat. I usually am the designated "flopper tuner" and can tune everyone's floppers on the boat in a couple of minutes. I highly recommend this tool and I left it public for anyone to print it for themselves.
https://youtu.be/dz_QhcKSDtk