Thinking about side effects is only a part of the issue. E.g. I use immutable classes with builders in Java almost dogmatically.

Run-time cost thinking is something that is never brought up when FP is contrasted to other forms.

For example, in CLRS when you look at run-time of a function with loops, it's really really easy to just look for the loops.

But with FP... people like recursion. Solving recurrence relations is Hard Work (tm).