Apple has really improved the slow-motion quality of the iPhone with the release of the 8, 8 plus and X phones. The 1080p 240fps mode is not only detailed but has very few aliasing artifacts that were the most prominent flaw of the 7 and 6 variants of the handset when shooting at high frame rates.
While the samples are few and far between, the increase in quality is very noticeable with the added benefit of better dynamic range across the board in highlights and shadows. While more tests are needed it is safe to say that at first glance the iPhone 8 and for that matter the iPhone X cameras are probably the best video smartphones released to date when it comes to slow motion video quality. Well done Apple!