The biggest problem i currently see is that we cant use the full system. We have too many green stones (i have almost 30k which i cant spend cuse im currently busy saving dias), and on the other hand we dont have a SINGLE yellow stone, which means we cant obtain 100% of the stones CURRENTLY. And if you think about it, the whole system is based off of rng. First you need to farm a card, which has a "low" dropchance. (If you like to) you need to roll the stat you need with a high value. Then you need to buy different stones to add even more stats, reroll the stats or even change the value. But imagine it wouldnt be all random. People, both the player that farm the cards and the people that buy the cards, would be in (lets say) 2-3 months done with all obtainable cards, each card with perfect, highest stats. 1 card = 40 cards if you compare the stats (i didnt do the math). Its fuckign hilarious broken on the long term when you think about it. all those RNG's make it harder/more expensive to obtain the perfect card for you. And with that, it would mean not everyone would have perfect cards. Only people who have the wish to get those cards which will take alot of effort.
Personally, i like the rng, because it gives you wonderfull feelings once you got your desired stats rolled. Obviously, it can be more frustrating than enjoyable, but once you got your first 10str card, go to the shop, buy a reroll stone, and get an additional 28.6 patk, its a really good feeling and you feel like all the work payed off. And since you can obtain everyone with time (even tho i doubt its possible without personal shop) for free, its not as bad as plussing stones, which you can only obtain for diamonds/a shit load of rubis. Yes, i can exchange gold/mems for diamonds, but in the end you pay with diamonds. You can farm 60 cards daily and try to get the best first stat, then pray and start rolling with the stones you bought from disenchanting the other 59 cards and some gold.
and here we have the lucky ppl and not so lucky ppl. I didn't dropped a single 10 attribute card myself within about 200 cards or more. With such luckyness you can also drop random values on stats in rofl and/or random values on golden gear.
I'd better pay an amount of diamonds for which I know, what I will get. Some plusstones with which I can plus my equip (I also use perfect stones).