I don't think moonflair would break the game, or anything. I just think it's a really, really boring item that allows champions to circumvent what should be an important choice. You should have to choose, at any given moment, between building offense/defense/utility. If you want a mix of damage and tankiness, you can build a few offense items and a few defense items, but you still have to choose, at any given moment, which one you want to work on next.
Items that give a mix of offense and defense have been nerfed time and time again, especially if they are rushable in lane, because they are hard to balance and ultimately boring. Abyssal got gutted in S2(? maybe 3?) for being able to do this. Atmas was gutted then removed from the game entirely. Sunfire has been nerfed repeatedly, particularly in the early game. Cinderhulk has been nerfed multiple times since its release. Frozen mallet was gutted.
At the end of the day, champions should have to choose between building offense and defense. Moonflair circumvents this choice, by being an almost 50/50 split of the two. Not only does it give an even mix of offense and defense, it gives an even mix of armor and MR, AND tenacity.
Moonflair's mix of stats means it will never be a bad purchase on any AP champion at virtually any point in the game. It might not be the best purchase, but it will always be a good one. This is a sign of a pretty uninteresting and ultimately poorly designed item.
It was released as a band-aid fix to help mages out in TT at a time when they were struggling. But the challenges squishy mages faced in TT don't exist int he same way on SR. SR doesn't need this band-aid.