There didn't seem too much when I played, albeit for a limited amount of time. At no point during leveling did I ever run into inventory issues for example, the base inventory is large, and monsters don't drop stupid vendor loot shit and such. You do get a lot of armor/weapons, but you can disassemble it on the fly so whenever you're full, you open the thing, select the rarity for auto diss and just remove all of it. It leaves a couple of stackable vendor items from these and you just vendor that next time, or not, I leveled to 50 without selling them.
In late game it might be more of an issue but there's really a lot of base space but when I see videos, they don't have that many items a lot of the time unlike some other games so it shouldn't be a problem(unlike say when I played DFO and you HAD to buy inventory slots to play properly, although they gave them away for free every month/month and a half). You also have the pet inventory, which does require you have the "sub" thing like all F2P korean mmos do(pay x for 30days of various advantages, some not so great but some are nice so worth having at least early on, you do get a month with pack, crystalline aura is what it's called in our version), and then you have a bank too iirc? And you can transfer a bunch of shit on alts just for mules.
The card video actually explained that, you can put excess cards on an alt until you expand card inventory. Seeing as how most cards aren't part of decks anyway, you really only need a few cards equipped at any given time. The passive collection bonus is stupid shit that's irrelevant until you're like a year in and have nothing else to do but farm 0.06% dmg for hundred of thousands of silver.
Another thing is for this type of game, they often have events giving this sort of things for free every few months. Like you'll get char slots, card inventory, normal inventory, pets, mounts and so on from events eventually. And yeah as mentionned it uses the blue crystals for unlocks, which you get from various sources naturally without having to pay. You can pay to get more but you don't have to so it's not so bad.