I also think it is a bit overpowered. Mainly, because the Phase Dragon + Shard of Wisdom combo involves only one element and Ether could have stood alone well 'tll now without this shard.
However, I can see bigger problems with it. First, Mono Ether has been a fine deck so far without the shard. Second, any other uses of Shard of Wisdom than targetting Ether creatures requires a higly complex combo (as others mentioned it). Third, it's typically a card whith is overpowerd in the hands of the AI. A 3x mark+Double card per turn arena deck can easily accumulate the quanta for the dragon (the Shard itself is ridiculously cheap), most probably it'll always have enough quanta for the maximum 12 Phase Shields to chain them, and in most cases, enough HP to sustain the "occasional damage" 'till the Dragons do their job. The only reasonable counter is Jade Shield which is a costy one anyway (5/7). Reflective Shield is rarely found in decks, since Light has better shield options whil'st Multi/Rainbow decks would need to add an extra card against the Shard which is almost useless in every other situation.