https://wiki.mozilla.org/Oxidation
"...
Rust Weaknesses
One major issue with Rust relates to personnel.

There is a wide variety of experience levels within Mozilla, for both coding and reviewing.
Rust's learning curve is steep at the start, which can be intimidating. (The new borrow checker released in Rust 1.31 helped greatly with this.)
There are also technical challenges.

Compilation is slow.
Crossing the C++/Rust boundary can be difficult.
See "Blockers and obstacles" below for more details about work being done to remedy these weaknesses.
..."