I've had completely the opposite experience. Have worked on several medium-to-very large Xamarin projects (4 developers up to 60 developers) and rarely have I seen code sharing drop below 50%, with the sweet spot being around 60%.
2 years ago, Xamarin was buggy as hell. Today, (IMHO) it's quite stable.
Edit: downvoted for sharing my experience? Stay classy, HN.