There was an interview with the lead mechanics designer (in Italian) here: https://multiplayer.it/articoli/baldurs-gate-3-intervistata-lead-system-designer-larian.html

Amongst a number of positive changes, was this line that scared me: "The other thing we've changed is how magic users use spell slots, making it less punishing to level more than one magic class. One issue with multiclassing is that if you multiclass early in the game, you don't get strong abilities like "Fireball" at the same level as a "pure" class, but we wanted players to be able to multiclass from the start of the campaign, without having to necessarily wait for the advanced levels, so it was the case to revise the use of resources a bit."

This seems to suggest that level restrictions (and perhaps only highest available slot) matter when learning spells. This is a pretty significant buff to casters. This may imply that on a pure caster (e.g., cleric, sorcerer, druid, bard) a single level dip in Wizard gives full access to the Wizard spell list (because wizards can scribe spells). They would still perhaps be limited by the number of spells they could memorize, but given that you can basically change your memorized spells outside of combat freely, it's not a huge deal. This is further complicated by classes that instantly learn their whole spell list. So a level 4 wizard takes a level in cleric. Does the character get fireball AND the first 3 levels of cleric spells?

How does this work with the major power gate for martial characters, extra attack? Does a fighter 2/ barbarian 3 get extra attack? Do you still get ASIs on 4 8 and 12 no matter what? If some (or all) of the costs of multiclassing are removed, the optimal build will probably be 4 classes (e.g., to get intro benefits of 4 subclasses). How does one balance difficulty for that? The power difference between a pure class character and a hodgepodge with no downsides is going to be huge.