I think both are true. It is true that we aren't good at functional programming because we are not practiced. It is also true that functional programming makes some things intrinsically harder to do than in the imperative case. This is because modern functional programming gains its power by restricting the programmer, and building on those restrictions.
(Sometimes I get in trouble when I try to distinguish between "new functional" and "old functional", but despite superficial similarities they are diametrically philosophically opposed. Old functional, embodied by Lisp, works by trying to empower the programmer, and builds on that; new functional works by restricting the programmer and building on what guarantees we get from those restrictions. Haskell and Lisp may share some terminology when it comes to list manipulation but I would actually put them very, very far apart in my grand map of programming languages; they take one step together, the first lambda calculus step, and then immediately begin sprinting in opposite directions.)
Lisp was always multi-paradigm and supported functional programming.
I'm also not sure I'd go as far as to say "restricts" as much as demarcates. Composing existing code has been vary difficult in imperative languages (practically impossible when you have to deal with things like memory management and/or threading).
(Sometimes I get in trouble when I try to distinguish between "new functional" and "old functional", but despite superficial similarities they are diametrically philosophically opposed. Old functional, embodied by Lisp, works by trying to empower the programmer, and builds on that; new functional works by restricting the programmer and building on what guarantees we get from those restrictions. Haskell and Lisp may share some terminology when it comes to list manipulation but I would actually put them very, very far apart in my grand map of programming languages; they take one step together, the first lambda calculus step, and then immediately begin sprinting in opposite directions.)