I've been wanting to use it for some b2b applications, or maybe for some reports/charts. Just in places where it makes sense to need that power, but you'd want it in a browser.
Unreal and Unity web builds are not production-ready yet. The cross compilation can be brittle, compile time is huge, the resulting assemblies are large and memory consumption is just too high.
This leads to many, many problems, which are hard to diagnose. Getting games to work on mobile browsers consistently is nearly impossible. For example, last time I checked, Chrome on Android disables WebGL by default and limits memory to 64mb.
Unity is working on "Unity Tiny", but it's in alpha state. Frameworks like Three.js are better suited, but more barebones. Still a long way to go...
EDIT: just to support my claim, here is an example of a WASM runtime implementations that run 4x-6x faster than others: https://github.com/perlin-network/life#benchmarks