Fatebringer is a far more efficient killing machine, especially on the raid. The Firefly perk also disrupts the forward progress of mobs when you get a crit kill, which is very often. But the biggest pro of Fatebringer over Mythoclast is that it frees up the exotic slot for far superior exotic weapons, those being Ice Breaker and Gjallarhorn. A few Ice Breakers with weapons of light on Atheon end it quickly, and Gjallarhorn with 7 or 8 rounds during confluxes makes easy work of hobgoblin mobs and Praetorians. If Mythoclast had 250 rounds in reserve and got the 56 round mag back, it may be viable. It is really good for getting the fusion rifle bounty, but that is the only place it outclasses as a primary.