>>11.Garbage collection adds unpredictable latency.
2.High memory use.
3.Reliance on recursion and its optimizations
4.Requires concentrated mental effort to understand, code isn't self-evident.
5.Different terminology and culture of programming that prevent newcomers.
6.Functionally impure/unsafe code is often required for performance and its essentially what the "pure functional code" is built on.
Reading GHC runtime source code is reccomended to understand on what foundantions the "safe&functional" languages stand.