The shards used only raises the chance of that element being the one that become unstable if it happens...but all 6 elements can become unstable at every synthesis.

I often carry most of brands with me...brand of ice, lightning and earth specially. Ice and lightning just mess with progress rate and earth seems to be exploding way to easily since 1.17a

About the dev post...no idea what he was talking about lol! Maybe it was like this but they changed after...or he just forgot and "often" there, so the correct would be "Naturally, the element which becomes unstable is often determined by the type of crystal or shard being used."...this way it does make sense.