Yrden made everything trivial for me, the zap on enhanced Yrden stops all attacks in progress and knocks down any flying creatures. In the later game it doesnt hit as hard unless you fully go into the tree but i had no issues dispatching most everything besides a couple bosses by just dropping 1 enhanced Yrden and a bunch of lesser ones to slow them down. The game honestly gives you a decent few choices in how to dispatch most enemies.
I could be wrong, but I don't think there are any missable cards. The one everyone was concerned about (missing White Orchard inn keep) was added to the merchant you saved from the griffin at the very beginning of the game.
Somebody correct me if I am wrong, but aren't there actually quite a few missable cards? For instance, the 3 matches you can play while at the Ball with Triss? Those matches are not available after that quest concludes. Numerous other occasions where you can play an NPC but only during the questing window where they are available (or still alive even).
Merchant cards are random but there's only a limited pool. You get one card per merchant, but it's a random card that you haven't yet gotten from a merchant. There are more merchants than cards so you will eventually play a new merchant on not get a new card.
You get a lot of tools to handle bigger packs, at least in the melee and signs trees. Some of them are cheesier than others, but you have options nonetheless.
I haven't played the latest AC games, but they intentionally crippled the AI to just stand and watch while you were dispatching somebody else. There are some differences, if you are 1v1, then there is rarely a reason for you to get hit by anything, including bosses. Getting surrounded is a death sentence, unless you kill stuff in 1-2 hits and cut your way out. Most of the skill for people here is in how to approach the encounters, the controls themselves aren't hard to pull off, especially if you are patient and back off after 2-3 hits. I was missing some enemy type that would heavily punish attacking out of the blue, I could usually take my chances with fast attacks and break their offense.
Basically yeah usually run up and slaughter them with quick strikes, or one shot low level mobs for treasure caches or whatever. Giants/trolls/contracts of level or higher I might burn through all 12. Or one moon dust for wraiths, it works like Yrden trap making them whole and taking a lot more sword damage.