- By default BLUI runs at 60fps vs Web Browser 24fps (can be changed in C++ slates but not in BP AFAIK)
- Run-time js events and callbacks
- More input control (mouse, kb, etc)
- PDF/Download support
In the past it also used to be more up to date in CEF versioning, but the built in has largely bridged the gap. That said if you got the inclination you can use a custom CEF version to include custom codec support or other features that are usually not built by default in CEF.
But that said if WebBrowser fits your use case you should use that first, only if you need a more custom use case should you consider BLUI nowadays.