Is Conflux a broken castle?

No, the Conflux is definitely not an overpowered castle. Honestly, it annoys me sometimes, but when I get tired of playing other factions, I don't mind playing it just for a change of pace... As for its strength, this castle is well-balanced – it doesn't significantly outperform or underperform compared to others. Even if we consider the Grail, for example, placing it in the Tower or the Conflux – both have their advantages, and it's hard to say which is worse.
P.S. Personally, I would choose to place the Grail in the Conflux.