Black Legion has a few of everything. That's why they're in the game; it allows for player growth.

You're right, Night Lords are fragmented into warbands so you can have any sort of Night Lord you like. Acerbus for example.

The Iron Warriors and Night Lords both have their share of marines who have devoted themselves to the path of a single chaos god, as have the Black Legion and the Word Bearers.

They may have different views on how useful or positive that is for the individual and/or the legion. If anything, the Word Bearers are probably the most reluctant to allow it to occur of the four, but no-one is above using those brothers as tools in battle.

On the other hand, a legion sworn to one god wouldn't allow its members to follow the path of any others, or the path of Chaos Undivided (those who do would have left to become renegades or joined the Black Legion).

So from a gameplay perspective, its a darn sight easier to pick four undivided legions and use the same mechanic for choosing one god, all, or none.

The other rational option at launch would have been to pick all four sworn legions and balance classes accordingly, but it would have been more work - and chaos would have become more restricted to role by legion selection than the other factions (unlikely to be considered a good thing by chaos players, unless each role has only one 'good' build, which would likely be a terrible thing for all players).

maybe GW has said this is the legions you can have for now, but im ok with the ones we got. will be good to see the NL in game but will probably play as BL

I just hope that when they release the other Legions that your character in game will be able to switch over to that Legion by switching armor colors or whatever they are going to have in game to show the different Legions.

Maybe these Legions somehow fit the story we have no clue about yet.

Either way, what is disappointing to one person is exciting to another because their chosen Legion is represented.

Although My personal legion is not in there (Thousand Sons), I think this will work fine.

You're selling the undivided legions very short, OP. Each chaos undivided legion does have a distinctive character if you know anything about them.

Personally I'd say it was the Cult legions that lacked distinctive roles, seeing as Thousand Sons are all either Sorcerers or Rubricae, all of the World Eaters are Berzerkers, Death Guard are all Plague Marines, and most of the remaining Emperor's Children are Noise Marines. There's not much of a choice of role there for a player joining one of those Legions as to what they could play as - they lack variety.

You talk of the Legions as if the Legion you will pick IS your class/role choice, and from what we've seen of the design of Eternal Crusade so far it isn't. Your role can be changed upon every respawn after death (just like you can change role in the Battlefield games to a different kit, like Assault or Support) - so in whatever Legion you choose you will likely have the choice of playing as a chaos marine, a havoc, a raptor, and whatever other specialists the devs include for chaos (e.g. loyalist marines have Tactical, Assault, Devastator and Apothecary confirmed and a possibility of Terminators, plus Chaplain as a hero class).

You couldn't do that with the Cult Legions in WH40k. You honestly think the World Eaters still have Havocs? No, every World Eater would have to spawn as a Berzerker, so there goes your gameplay variety and choice of distinctive role if you picked World Eaters as your Legion to play but sometimes felt like being a longer ranged havoc for a change.

At least with the Undivided Legions there are choices as to what you can play within them. Just the fact that the developers are letting us only choose from Undivided Legions should tell you that Legion choice alone is not meant to have any effect on gameplay - it's a thematic choice. The roles you focus on playing within your legion will affect gameplay, not the legion choice itself.

I personally love the fact that the devs chose (with the exception of the Black Legion) the warbands that A.) are very distinctive and B.) Not of the main "4". I think they were wise not to include the major four powers at launch - I personally will be more than likely abandoning the Emperor's light (no Black Templars) and playing A Night Lord or Word Bearer, solely for the fact that the art team has promised that the NL's armor will flicker in coursing lightning. If that's not distinctive enough, I don't know what it is. IT'S FREAKIN LIGHTNING.