There was likely an agreement with final fantasy on why it couldn't be in the mythic shop. Plus because it is universal its a bit better than others so even if they could my guess is the cost would not be the normal mythic shards. The reason being is once you have a universal one it kind of kills the desire to get one for another legend.
As to whether the sword is a good heirloom vs others, I disagree with xhelperz. I own quite a few heirlooms and it's pretty nice. On all heirlooms the inspection animations are a cool maybe look at them once thing. What matters for me is the look and how the heirloom reacts during normal carrying and pulling. The sword has great animations in tha regard.
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