js_of_ocaml has deep integration with the DOM, event handling, and tyxml (to emit HTML5 in a type-safe manner). It had a fine-grained reactive DOM before Facebook React was even a thing... Bucklescript could add those features, but it'd take a lot of work, and I'm not sure there would be much benefit over what js_of_ocaml offers.

The sweet spot for Bucklescript seems to be in exposing bits of computation-heavy logic through OCaml. The reimplementation of a 55k Immutable.js in 899 bytes of compiled JS makes me go oh-damn-did-you-see-that! On the other hand, js_of_ocaml has rather unparalleled support for building UIs -- which probably makes it a better choice if you want to do your whole web app in OCaml, especially in combination with frameworks like Eliom.