More relevantly, if you want to achieve happiness, achieving boredom constitutes failure, but achieving sadness constitutes an even greater failure. If advice that prioritizes avoidance of boredom with ample resources has anything going for it, it's that boredom is a more stable attractor than sadness.

This doesn't constitute a refutal, but when I'm in a state of boredom, and don't feel that I can achieve happiness right then and there, I will often seek something out to make me sad, or at least angsty. I think this is partly because my brain has a much greater tendency to get "stuck" in boredom than to get "stuck" in sadness... Something about the intensity of feeling sad makes it just more interesting than boredom, and I can pop out of it pretty easily. (I did meet someone who was surprised that I did this, since he found his brain had a greater tendency to get stuck in states of sadness that would become depression.)

On a more general note, whether or not happiness/sadness or happiness/boredom are opposites is based on how you define the word 'opposite', and I don't think it's necessarily pertinent to deciding which one is the 'greater enemy.' Sadness and boredom are both signals that something is wrong with your current situation and you need to start behaving differently to fix it. Sadness is probably the more urgent signal, in that it tells you something about your situation is actively deteriorating or has been permanently broken, whereas boredom is just a signal that things are okay currently but you should probably be exploring a wider range of options. I would like to be capable of experiencing both, because just like humans incapable of feeling pain are pretty dysfunctional, anyone incapable of either sadness or boredom would likely find all of their decisions affected by it. Shutting off the negative-emotion signals doesn't achieve happiness anymore than shutting off pain receptors instantaneously achieves pleasure.