So, for competitive play, there are a few lists which you will need to be ready for.
1 death guard with hordes of fearless plague zombies and mortars
2 mortarion + Magnus + Knights
3 Altioc eldar. Typically with fireprisms and deep striking Wraith guard
4 Ultramarines parking lot with guilliman.
5. Turn 1 charging genestealer hordes.
A lot of the big lists you will run against have lots of vehicles, or rely on single model units to do a lot of their work.
Typically, I think the nightlords leadership stuff is a gimmick, which is not something you can rely on to pull your army through.
Now, I'm thinking lvo level competitive sort of stuff here. I'm not sure what competitive level you're expecting to see.
There are a couple of things I see lacking in your list.
1. Screening units. These fulfill two roles. 1 they stop you from being charged t1 by a variety of armies, and 2, can absorb smites from smite spam armies.
Chaos have some great synergy with cultists, and if you're not running some sort of cheap screening unit, you might find yourself in a lot of trouble.
2 long ranged firepower. Most of your damage is delt via rapid firing plasma guns, often. By deepstrike. Plasma is great, but a good player will deploy with their screening units in a way that you won't be able to rapid fire anything (but chaff) when you drop in.
You need weapons which can do solid firepower at 24" minimum, and ideally longer ranged. You need to be able to reliably kill a daemon primarch, or a knight in one shooting phase.
What kind of missions are you expecting to play? Itc?
Now, these are the two biggest flaws I see with the list. Granted, it's a strong mid-table list. Dropping plasma is still strong, but it won't be reliable. There is a reason you don't see lists like this winning major tournaments.