Fidelity wise speaking, the CupertinoActionSheet isn't pixel perfect along with the native one and also there are some key features that are missing. Below are some of the findings I've gather that should be enough to make it perfectly match it.

Environment:

• Device: iPhone X
• OS version: 12.3.1
• Font scale: Set to minimum

Flutter screenshots ar on the left, iOS native on the right

Font scale factor shouldn't be allowed below 1.0 and bottom safe area is wrong

The bottom safe area has more pixeis in the Flutter that it should, also, the font scaling factor, in iOS native, doesn't go below 1.0 whereas Flutter allows it, it should be max(1.0, fontScaleFactor).

When system font scale is set to maximum, the sizes don't match properly

Looks like Flutter is scaling a bit more that it should. Fixing this, can potentially fix some other wrong scaling across cupertino widgets.

Opacity when selected

Despite the fact of having different backgrounds, it looks like Flutter is using a different opacity when selecting the item in the sheet. Plus, native allows you to keep it pressed while navigating up and down without raising the finger, providing a light haptic feedback when doing so. Flutter doesn't allow this at all.

Flutter doctor

[✓] Flutter (Channel unknown, v1.8.0, on Mac OS X 10.14.5 18F132, locale pt-PT)
    • Flutter version 1.8.0 at /Users/miguelruivo/DevTools/flutter
    • Framework revision 2fefa8c731 (4 weeks ago), 2019-07-01 11:33:22 -0700
    • Engine revision 45b66b722e
    • Dart version 2.4.0