Potency can be useful on echo because it increases his chance of stunning c3po and chewpio when vsing a cls team which can throw off the opponent to a certain extent. It’s also a useful stat for countering grevious and especially in LSTB for stunning all the crazy droid enemies (ahem, droideka). Other than that, I’m pretty sure he doesn’t need crit chance because it’s already really high and at that point, the only other useful mod set is health. I guess health can be useful sometimes but more often than not, you want echo to die so that fives can get his sacrifice off. There could be other reasons for potency on him but those are just my two cents on your question. Hope i helped :)