JS' semantics are pretty much opaque when staying in Purescript. Of course all bets are off when using the FFI, but then again the same goes for GHCJS.
One major thing in favor of GHCJS is the ability to share code between backend and frontend, but that's not a concern when e.g. the backend is written in another language anyway.
EDIT: Also, Purescript on the backend (targeting node or native code) is slowly becoming a thing as well, as this is what this submission is about :)
well, for anyone who's been in this situation, it should be a concern, since it's incredibly easy to get out of sync between backend and frontend (e.g. model, validation, etc.).
Ideally we could all code in preferred-language; target all major platforms using a single/universal UI framework, and call it a day...but reality is messier than that, we're often forced to code apps in multiple languages, multiple UI frameworks (if targeting native iOS + Android, and browser/PWA), and hope it all comes together such that the house of cards stays standing through language/framework/platform upgrades.
When I tried using ps-concur or some other ps gui libraries, I didn't feel inclined to keep progressing. When I learnt reflex with ghcjs, I built stuff, and I kept finding new things I wanted to build.
The best way for a Haskell to target browsers at the moment is via ghcjs. Hopefully one day it'll be with a ghc-to-wasm compiler.