Mutliclassing in 5e is a half baked optional rule, the land of broken (mostly very underpowered) combos. One good reason for Larian to avoid it is it gives players without 5e system mastery a whole lot of trap options to fall into.
for example, while Wizard/Cleric combine very poorly by cutting off access to high level spells.
Actually, if you include wizard, it's not so bad. Caster levels stack for purpose of spell slot allocation, but they don't for spells known/prepared. But wizards can still learn the higher level spells from scrolls. It's things like cleric/druid that are completely locked out of the higher level spells. But to implement separate list of spells known/prepared for multiclassing would require a compete do-over of the BG3 character sheet. And you need to find room for up to 9 separate spell lists. Unless you cap the number of times you can multiclass of course. But that's not PHB.
And Larian have created additional problems for multiclassing in the way they have implemented certain features. For example "Paladin Oath Charge" should be Channel Divinity, and share a pool of uses in a cleric/paladin multiclass. The way Sneak Attack is implemented would not interact correctly with Extra Attack in a Fighter/Rogue, and Barbarian/Draconic Sorcerer armor class would be completely borked.
But it is also a very popular feature that Larian have stated they were going to add.
It's easy to promise things before you have fully assessed how challenging they are going to be. As a project manager, people making the client ridiculous promises is the story of my life!
