Apparently some people's spit doesn't work very well as a defog -- not enough surfactant molecules in it.
Most of the accessories I've bought and disliked have been backup lights which have failed. We had an Ikelite switched light that flooded, and a Dive Rite 1W LED which flooded, and two Oxycheq Raider IVs that, although when they worked they were the best backup lights I've ever seen, were so viciously plagued with problems that we gave up on them (lights failing and then their REPLACEMENTS failing).
Another thing I bought that didn't work out as planned was my spreel. The initial problem I had with it, which was a rusting issue, the fellow who made them made good by sending me a new screw to replace the one that corroded. I had to take some line off the spool, because it would get too full and jam, and I finally gave up on it when the tensioning mechanism wouldn't work well enough to prevent line from unspooling. Since there is no other way of securing the line, the device became unusable. I was really disappointed, because it was an awfully good idea.
I guess the surf shoes to go over my Fusion would count as "accessories". They worked great for walking and light diving, but came off my feet in a cave, so they got jettisoned.