In a different RPG with traditional companions you'd 'only' have to put in development time and resources to let you interact with the companion and experience their companion quest(s) through the eyes of the player's custom character. Basically making the player's custom character an observer to the events that happen to the companion.
The additional work refers to how Larian has to make playing as one of the origin characters interesting enough in order to justify it being an option. This means that some dialogues will require unique options only available to that specific character. Some problems and encounters will require you to be able to handle them in a specific way that'd make sense for the character in question. Some cutscene sequences like
the dreaming sequences in BG3
will have to be recorded, written, animated, scripted and voiced just so the player can experience them - essentially making it 'game content' that exclusively is available if you picked that specific origin character. Now multiply this by every unique encounter in the game and then multiply it again for every origin character who gets unique interactions. It quickly adds up to a lot of extra time and resources. What the people you're referring to are concerned about is whether or not this is a sound investment since they'd rather have the time spent developing or refining other aspects of the game.