Even though the system sounds op in its making, I am thinking of the cons first.
1. Its going to be worse then the grand gems system. Getting the right combos in the slots is going to be near impossible. This is especially since all slots reroll together.
2. Time factor. If this system again requires reroll times which take like 8hrs or so its going to be really frustrating. F2p players have no choice but to put items when they are at work or asleep.
3. Awakening gem is again a story of collecting ingredients, thereby filling up crafting inventory which is already hampering play even at 500 slots & crafting times for the awakening gem in an already limited crafting slots situation.

Not trying to sound negative here but the whole jewels, crafting inventory, crafting costs, crafting slots, other ingredients that take up space & time also, is getting more & more cumbersome by the day.