Most units are good. There aren't a lot of traps in the codex.
The real winners:
-canoptek wraiths used to be pretty badass in the previous book, with entire armies revolving around them...and then they BUFFED them in this book.

They are now one of the game's top quality units, and the canoptek harvest formation can make them even more silly. (Like, it takes entire armies worth of concentrated firepower to down a unit of harvest-wraiths, for example it takes 54 bolter shots from bs4 marines just to remove a single wraith.)
-Canoptek spiders may seem a little lackluster until you realize that most other armies are getting an unequipped small time character or a decently equipped veteran sergeant for the same price that you're getting a t6 3 wound 3+ save monstrous creature. The fact that he can spawn scarabs is pure bonus. As above, canoptek harvest covers him with a nice cheesy sheen.
-Destroyers are marine killers with t5, 2 wounds,
JSJ, and preferred enemy (everything.) Again, one of the decurion formations turns them up to 11.
-Not only is a destroyer lord insanely tough, but he can add preferred enemy to whatever squad he joins.
-tomb blades aren't as tough as some of the other options, being "only" t5 1 wound, but they have a cheap upgrade which can make their
TL heavy bolter gauss weapon ignore cover, much to the dismay of every camo cloak scout in existence.
Really the things to not be excited about in the army are the vehicles. They are just kind of average quality. The real kick-your-teeth-in competitive lists tend to be decurions of warriors and immortals marching forward with canoptek harvests and destroyer cults mixed in the ranks, and one popular centerpiece is a lychstar, made by taking several lychguard with scythes, a lord with solar staff and res orb, and then another lord (like say the destroyer lord from the destroyer cult) with veil of darkness and another res orb. In that set up, you have a unit that can get in their face turn 1, and make themselves effectively invincible for 3 turns, plenty of time to spread out and multiassault several things.