All the legions are viable in 30k, so you don't need to worry about picking a weak one. The armies in 30k are a lot more balanced than in 40k.
There are no rules yet for thousands sons in 30k, so it's hard to say what they will be like, but I expect there will be a lot of psykers. Until a legion has its own rules, there is a generic army list you follow, and then choose one of a few USR's for your force.
Because Marines are not immune to fear in 30k, night Lords have a lot of stuff which causes fear. They are one of only 3-4 legions which are able to deepstrike their terminators, which gives you a lot of options too. Their units get Bonus in combat if they outnumber their opponents.
They have a couple of unique units, which are the Night Raptors and Terror Squads. Both look really cool.
Because of how your HQ's work,and the the Path of War rule, there are a large number of diffrent ways to build your army. Each legion has a couple unique ways to build their army, but there are almost a dozen other ways shared by all the legions (each with advantages and drawbacks).
Want a dreadnought army? Go for it. Want lots of jump troops? Sweet. Want to drop pod in? You can do that too. This is a very small example.
There are also rules for shattered legions. Small bands from multiple legions working together. You could run a shattered legion of thousand sons and night Lords. Gives you more options, but there are drawbacks to that too.