That wasn’t my experience when I tasted some from a neighbor’s patch, they were very sweet, with an interesting flavor very unlike most raspberries. I’ve eaten lots of salmonberries on hikes and walks through parks, and never had one even half as sweet as these were:
I will say that they had very little tartness, but the flavor reminded me of tropical fruit and there was plenty of sugar. But they were growing in nearly full sun, so maybe that’s part of the reason they were so sweet.