I don't plan to multiclass, but I thought the downside was you will not be able to get to high level in the classes you pick. I could be wrong. Anyone know?
The level cap is 12 for single class, so 2 classes picked for multiclass would be 6 for each and level 4 for 3 classes?
You don't need to take the same number of levels in both classes, it's common to take a single level from another class to get some proficiencies or class features.
The biggest negative is that you postpone cool class abilities, and you cannot get the best ones from higher levels. This also means that you could mess up the character if you don't plan the build before starting, but this isn't a big problem in this game as you can change everything later.
In the interview they mention a specific case of taking two spellcasting classes, that's what I was wondering about.
When multiclassing, the levels of your classes and subclasses that normally have access to spell slots are added together in a weighted formula, and then the overall spellcasting level is used to determine how many and which levels of spell slots you will receive.
Granted, we don't know what that "weighted formula" specifically is, but isn't that just how 5e normally is? A level 3 wizard + a level 2 ranger is treated as a level 4 caster when determining spell slots?