Yeah I remember it's been like Raldo said since forever ago.

Q. What steps can I take to ensure that the element of my synthesis does not become unstable?
A. The element has a chance of becoming unstable whenever any step of the synthesis process ends in failure. It is at its most susceptible when the glowing light representing the craft is colored, and is at its most stable when this light is white. Additionally, the chances of an element becoming unstable increase as a synthesis attempt nears completion. The element which becomes unstable is influenced by the type of crystal or shard being used.
http://lodestone.finalfantasyxiv.com...f271d7a0a84d5f

I think "influenced by" doesn't mean limited by. But maybe it plays a factor, like, if you use fire shard, it might have higher chance to get fire element unstable, etc.