They aren't really bad, they perform pretty well but They don't show consistent performance and they tend to throttle much quicker than the snapdragon ones and battery life also appear to be better on snapdragon version of the same phone with exynos chip.
Not really true. Newer Exynos processors, like the 1080 (China only) or the flagship 2100 are on par or even arguably better than their SD counterparts.
