Brian, I'm bad at being able to explain flavors. Good dragonfruit are great while there is a lot of junk out there. I don't think I'd ever buy one in a store again as they are almost always bad, though folks like Gray Martin (friend in first photo) hope to change that. Most white-fleshed varieties are bland tasting or at least fairly mild, but some can be pretty good. Gray has developed a nice white one that I tasted on Thursday (he's bred and trialed over 4,000 seedlings!) Most dark red ones are more acidic and not sweet enough for my taste but I have had them make some of the best smoothies ever (add a little sugar and maybe some lime juice). The pink-fleshed ones are usually my favorite and are quite sweet (brix 16-20) and mildly acidic and, possibly, flavor intensity of something such as watermelon (not that it tastes like a watermelon). The colors are wild, IMO. They are a challenge to grow in my climate though I did pick two fruit from a protected outdoor plant earlier this month. Many require cross-pollination (though Gray again is breeding for self-fertility). I have been toying with the idea of putting up a high tunnel and growing commercially, but maybe I should have my head examined to take on yet another project.
Dragonfruit lose acidity while being refrigerated so that may be helpful to dark red types but harmful to others. If you've had bad impressions from a fruit from a store, it's probably because of the source or storage. I generally prefer them refrigerated but the best ones are good at room temperature as well.