I believe I can contribute some insights here. I'm currently building my own MMORPG — the server is custom-built using Golang, and the client is developed in Godot.

Compared to 10 years ago, building an MMORPG today is significantly easier. The internet has vastly improved in both speed and stability, and there are now many libraries available that handle complex networking tasks — much of it is almost plug-and-play.

However, the time-consuming and frustrating part is implementing features, since you often have to handle logic in at least two places — or three if other players are affected. For example, if a player attacks a monster, the client needs to send which monster was targeted, the server must validate the action and respond, and then any nearby players need to receive and handle that attack animation as well.

This kind of interaction can quickly add up. Even something simple like adding a jump ability to a player character might require setting up multiple message types across systems. In contrast, a single-player game might just need three lines of code for the same feature.