The drop rates for everything in CotM are screwy, not just the DSS Cards. I've been playing for a while now (just past the 3rd boss) and in all that time I've had just two potion drops. Meanwhile I somehow have 13 Wristbands, 5 Magic Gauntlets, and 18 (!) Leather Armors pointlessly clogging up my inventory. It's one of a number of niggles that are really bugging me about the game.
The save points feel just a little too far from boss rooms to be convenient. Enemy placement sometimes feels a bit unfair (jumping up from one screen only to get hit by an enemy projectile you couldn't have anticipated). Sometimes it feels like the rooms have been cut and pasted to pad things out, particularly when you go through three or four identical screens only to reach a gated area you'll need to come back to later or a single MP Boost. And why on earth don't bosses drop any kind of health pick up? Not even a refill?
There was a development video for Bloodstained where Iga was being incredibly pedantic about the movement of the main character feeling "right" to enhance the sense of exploration. I just don't get that CotM. The walk is too slow, double tapping to dash is awkward but essential for some platforming... it just feels off. In some ways it feels more like a Metroid than a Vania, not that that's necessarily a bad thing, but it's not what I'm expecting.