Unity has many advantages for games and indie developers, and has a good community and a forum filled with questions and answers. Teams can also take advantage of the asset store, which host many features and and pre-made assets. Unity also has a team constantly developing and fixing the engine, taking engine work off of the shoulders of the game developer. Even with all of these pros I would still not recommend Unity for RotMG.
Unity cannot be obfuscated the same way that flash can. Unity prefabs keep static references to methods/variables within the scripts, so if they are changed, the prefab system would break. Although obfuscation is not the end all solution to hacking, it can be a simple step to prevent amateur hackers, and demotivate others from hacking.
Unity also has the issue that every game is build upon the same base engine. This way, a hacker can figure out a hack for an engine feature, then have it work on any Unity based game. i.e. They can modify Unity’s collision system to allow no-clip, and they can no-clip in most Unity games.
Unity is built to be accommodating to most games, and not to be highly optimized for a single game or game type. Unity does not allow fine-grained control over the rendering system, and Unity has a lot of overhead that would not be needed in RotMG (physics, lighting, etc).
Unity’s sprites are not highly dynamic. All RotMG needs is a renderer that draws Rectangle X from the sprite-sheet into Rectangle Y on screen, making a sprite only need to consist of the Rectangle data. Unity’s sprites are larger data structures that waste memory and cpu. In my experience, Unity’s sprites are fine if you create them all at load, but this does not allow them to be dynamically modified/created easily. Unity’s ECS system may help mitigate some of the performance issues, but it is still in beta.
My experience comes from recreating the entire Darza’s client in Unity. Unity is ever changing, and these issues may be dealt with in it’s future. For now, I’d suggest Monogame for its cross-platform capabilities and open-source code, allowing the whole system to be customized to your needs.