Zed is mechanically difficult to MASTER. However, in the lower elo ranks it doesn't take much to win as a Zed. This is for a few reasons: people don't know what to do against a Zed (harass while his w is on cooldown or poke when he goes to last hit a minion), people get the mindset of "Zed2OP" that they get too scared to do anything against him or don't think clearly when they do try something, and lastly, they don't realize that if you save your cc for when zed would want to go to a shadow, he won't be able to take it. Also, standing in between zed and his shadow, or if he ults you, dont run away from his initial position if you feel you will still die, this will make it easier for your jungler, top, or bot to stay on him without worrying about him using ult to escape.
The reason people such as Faker and Bjergsen are so good on Zed is because they realize the mindsets others have around a zed and take advantage of it. The realize the natural instinct is to run the **** away when he ults rather than move towards the initial position if allies are near. Also, they will incorporate things such as flash, shadows, and the movement speed boost from Yommu's and Blade of the Ruined King in order to dodge all skillshots thrown at him. Watch closely, their reaction times are on point, they will last second flash a skill while placing themself near the target to do damage, and rather than attacking while BoTRK active is in use, they use that short duration of burst to move from attacks. Things like this is what allow those two to all-in and pull out barely alive.