what I hate is how long it takes to cap, Re:CoM Marluxia's card drop + Doom combo (Marluxia at CoM was a piece of cake at GBA, but Doom and him making me drop my cards always got me at Re:CoM), you can't customize Riku's deck (only minor), Riku entering D-mode at the wrong time (like when you wanna use King Mickey to heal/reload, but D-mode makes you waste him) and the doors that more specific, instead of "X^" or "Xv"
what I love is how CoM got an unique gameplay, story, the duel system Riku got and Riku's evasive skills at D-mode
lol really? xD I remember getting it three times. xP I think it was pretty random, too. You could get it from regular fights.
maybe just my luck, but I noticed that it only took me like 3-5 (sometimes up to 10) battles at the final floor of Castle Oblivion to get the Golden Card, so I used that to farm and didn't take long for me to unlock the Treasure Room of each floor that way owo
so maybe try farming there, if you wanna hunt for that card?
NOTE: that was only at Re:CoM, cuz when I last played CoM so was I a kid that was too lazy to grind... XD