This question has two answers.
A1: Chromatic abberation in some wide-open lenses can produce very saturated blue or purple fringes around strong light sources adjacent to very dark areas, such as the headlights of the cars in the nighttime scene below. When these colors are mapped into the ACES color space, they can fall outside the gamut of colors that the ACES rendering transform is designed to handle. The resulting color clipping manifests itself as an unpleasant artifact.
The ARRI rendering (again, the ARRI crop is on the left) was specifically designed to be more forgiving in this circumstance, in that it provides a "soft falloff" to the color boundary; the ACES rendering on the right has a very sharp edge to the purple region surrounding the bright white of the headlight. See image 9.
Going from about a 5X blowup to a 15X blowup, in extreme close-up the difference is clear. See image 10.
It should be noted that, in a subsequent take with the lens closed down one stop, the purple fringing was much reduced, and the problem virtually eliminated. The point here is not to show the better handling of the edge case in the ARRI rendering, but rather that to demonstrate the edge case so that it may easily be avoided.
A2: ACES isn’t a ‘universal cure’. The difficult problems that come with near-monochromatic stimuli, mixed lighting, etc., are still present in ACES; there is no ‘free lunch’. If you’re aware of this, and avoid the ‘gotcha’ of being buzzword-driven rather than needs-driven, then do your standard pre-production tests, use our documentation, and you will get predictable, solid results.