A friend and I are currently running a campaign as Co-DMs, and we have noticed a couple of things about the encounter builder. 

In this scenario, I am not the DM of the campaign as far as D&D Beyond is concerned, so I do not have access to make any modifications to the campaign itself. That would be a nice feature, though, if you could assign the "DM" role to more than one person, but that's another thread. 

The important part is that I do have the ability as a "player" to create encounters and assign them to the Campaign. It accurately pulls all the player information and lets me set up the encounter properly. Awesome! I think it's a little strange that I am able to, as I can't think of a reason a player would normally need to create an encounter, but ultimately, I guess that's what I'm doing now, so I won't speak to that much.

The problem we noticed is that the DM cannot see the encounters I made, despite them being linked to the campaign. I think it makes sense that the DM of a Campaign should see all encounters made by anyone that are linked to their campaign. That way they can use them and manage them. 

I know it's still in Beta, I just think that would be a good feature to have.