* Comprehensive feature set
* Easy prototyping
* Easy modding
* No recompilation required
* Web full of documentation & examples
* Common skill for developers/designers
The main disadvantage is probably the performance hit of having a full-blown browser engine in your game, but it seems to work OK for AAA games. I believe Anno 2070 uses Awesomium for some of its content: http://www.posidyn.com/games/anno2070/anno2070-09.jpg
There are probably several CityBuilder servers, but I mostly played on the BTPro[1] ones. Everyone starts at the same time and the goal is usually timeboxed to 3 hours, after which the server resets with a new map.
I haven't played the game for a few years, but these goal servers kept me hooked for months.
[1] https://openttd.btpro.nl/