I did not have the feeling that the game is too difficult.
Sure, I died from time to time, but it would be boring if I didn't. Then I adjusted my cards and learned the timings for dodging attacks (not least for the trophy), played accordingly and always saved the ultimate for butchers and mutators.
In addition, I never played quests underleveled. If you don't just rush from A to B and are on the road a bit and do some sidequests along the way, you can get there quite well without having to grind.
All in all, I can say that if you take your time, the game isn't too hard. But if you learn how the game works (dodging, leveling up, leveling weapons, using the enemies' weaknesses to do more damage with the appropriate weapons, creating a character set that fits your playstyle, use the environment for your advantage etc. you'll be able to do a lot of things. You realize there's more depth to the game than I previously thought.
With all that in mind, the game becomes almost too easy.
I've played through Dead Island II solo and never got stuck or had to grind and I'm not a super duper rad player.
Can't wait for the DLC.