How can I know what prevents efficient lower level code when using FunctionCompile?

Question

When using the old Compile, I can use CompilePrint and ensure the resulting CompiledFunction doesn't use MainEvaluate. If MainEvaluate isn't used, the CompiledFunction is efficient. How do we know if FunctionCompile can make efficient code, and where the problem is when it can't?

Update

Below I see that some things the prevent an efficient result in Failure and a Message.

So should I expect that FunctionCompile was able to convert the program to lower level code if I don't get Failue and a message?