Unity provides a cross-platform framework for scripting games in high-level languages such as C#. However, it also supports platform-specific plugins in languages such as C, C++, Objective-C (for Mac and iOS), and Java (for Android). Developers can publish these plugins (and other assets) on the Unity Asset Store. Many published plugins represent a large amount of high-quality work, and buying one might be more economical than writing your own.
OpenCV for Unity, by Enox Software (https://github.com/EnoxSoftware/), is a $75 plugin at the time of writing. It offers a C# API that is closely based on OpenCV's official Java (Android) bindings. However, the plugin wraps OpenCV's C++ libraries and is compatible with Android, iOS, Windows, and Mac. It is reliable, in my experience, and it saves us a lot of work that we would otherwise put into custom C++ code and C# wrappers. Moreover, it comes with several valuable samples.
