For me, it's going from many choices in the base game to a limited choice in SoD. You could pick up Ajantis, Khalid near the start, Kagain and Kivan at/near Beregost, and Minsc at Nashkel before you even get to the Nashkel Mines. You could also include Shar-Teel if you're lucky to survive her encounter but for arguments sake, I'll leave her out. And that's not including the multi-class Fighters. If you're going to include Dorn with SoD, then he's also at the Friendly Arm inn but I think he turns up later to join you.
And who do we have for SoD? One tank which is Minsc. And Dorn later on the same map when you meet up with Khalid. That's it.
What about Thieves? Imoen, Montaron, Safana, Coran before you get to Baldur's Gate and later Alora and Skie. In SoD, you have Safana and Glint.
I find it quite jarring that you're forced into taking companions that you may not have wanted and didn't have in your party to begin with. At the end of the game, some of those companions you did have left your party anyway and I don't see why they couldn't have included all the companions and have them leave your party for similar reasons.