I Actually use a combination of both … I am currently using HTML loaded off an external web-application server (Re-Spawn and Flow UI) for my User Authentication and Player Leader Board, and Player Statistics and then use a UMG system in the game itself.
I actually find UMG relatively simple to use, granted I can do HTML development a last faster … but I prefer to keep my complexity out of the game and in the FlowUI system. This gives me a ton of benefits … but I am not going to go in to that now.