I'm currently developing a web app with a Haskell backend and a ReasonML (OCaml with a different syntax) frontend. I'd been interested in ReasonML for a while and so far it's been a great experience, but overall I wouldn't say I prefer it to Haskell. I've been using Haskell for many years and so the simplicity advantage isn't there for me. I also miss strongly some of the major Haskell conveniences like generic programming, type class derivation, higher kinded types, and yes Monads. I'm not a huge fan of the OCaml record system as it frequently gets confused and requires type annotations. In fact I've generally found OCaml to be substantially worse at type inference than Haskell, requiring frequent type annotations and cluttering up the code. Haskell's learning curve is steeper, but it is simpler in some ways, especially once you're familiar.

I think where ReasonML/OCaml wins hands-down is its compiler. Bucklescript (and I've heard similar things about other compilers, but can't vouch for it) is lightning quick, and the community has gone leaps and bounds to make it super easy to bootstrap (including with create-react-app). GHC can do amazing things but it's not the fastest out there, and the compiler for the JS backend is abysmally slow. The code Bucklescript produces is very high quality and performant. The library ecosystem is reasonably strong and I expect (but haven't done much testing) that because of the strictness and relative simplicity of the language, library code would tend to be pretty performant and easy to use. I've never found myself in OCaml wondering what string type I should use, or attempting to parse some absurdly complex type hierarchy in a library.

I think for a frontend project, to most programmers I would recommend ReasonML. For one thing, I think even most without experience with FP would find ReasonML to be a giant improvement over Flow (which infuriates me as a type checker), and it doesn't take a ton of time to learn. ReasonReact is a pretty sweet library and I prefer it to React even setting aside language choice. Compared to Haskell, its strictness means it doesn't require a special runtime, generate slow/impenetrable code, or hog tons of resources -- all things that are issues in frontend Haskell. For those who really want to use Haskell on the frontend I'd recommend PureScript instead (also you get the bonus of a kickass record system).

For backend, I think OCaml will get you up and running sooner, but in the long run you're going to find yourself missing the type system and language features Haskell has. GHC's speed on the backend is tolerable (especially if you use nix), and I think Haskell is competitive in performance to OCaml (perhaps better with optimization). Most importantly, I think Haskell is second to none when it comes to correct implementations of business types and logic, creating powerful and reliable abstractions, and eliminating boilerplate, and I think that forms a greater advantage in backend code.

EDIT: this turned into quite the essay...