For the most part the questions have one answer, the higher detail the worse the performance, bad performance is a very serious consequence, so you would have to pay particular attention to optimization early on to get the best performance, otherwise you end stripping your game in exchange for performance. There’s plenty of multiplayer games out there with high levels of detail and very large worlds, e.g battlefield or the division. But remember that The Division looked better while it was in development than when it was released because they wanted the game to perform as well as possible across all platforms.
Concerning dedicated servers there are plenty of reasons why people use those and it can probably help avoid lag in a multiplayer game, you can read this Fortnite thread for more info on why dedicated servers are used Fortnite de Epic Games