That is just FUD im my opinion. I havent har any issues with getting my applications working on multiple platforms. Have you har performance issues with C# on IOS?
Be practical. I've had to support wordpress on IIS before, it's always a lot of hassle not using the tech in its native environment. Same with MySQL + .Net. The EF, for example, played really poorly with MySQL.
I can't imagine using mono is for the faint hearted. For example in the new MS programming language thread [1] today someone mentioned they ended up giving up on the .Net GUI controls and using GTK# instead because it was so unreliable. Another mentioned that the transition from one SQL driver to another had left a lot of projects hanging in the wind with SQL breaking on mono but working on Windows.
I don't think that's a fair comparison. Rust doesn't come with a set of functioning, well-tested and cross-platform GUI libraries and database drivers either.
Running C# the language with the BCL on mono is pretty painless, and offers a ton of functionality.