The same thing that happens to every weapon class in the game: people complain about weapons being op and needing nerfs, so bungie obliges to those calls. In the case of fusions, people complained about the vex and pocket infinity the most. All you heard over and over was how those weapons were so op and making the crucible unfair. The complaining started dying down once people started cheesing atheon to get the vex easily, but by then bungie had listened to the calls for nerfs and went through with the action. Since then, fusions have been -blam!-ed, no matter how hard bungie tries to fix them. I think it's cause they never really had a starting point for them to balance from. They went straight with nerfs to the class, then nerfed it some more with hot fixes/balance updates, then they tried to balance it. Think they should just set all the guns back to the state they were in when the game originally came out, and then balance off of that. That way they have a starting point.