Multiclassing(assuming it's possible for origin characters) is more of a gray area and it depends how heavy you go into other classes too, but it creates a similar situation to respec where it's more like "Well Shadowheart technically still has one level in Cleric of Shar" but if we're being honest no, she's not really much of a cleric.
If we're going to argue about how good a cleric Shadowheart is, she's as good a cleric as you play her, since all her actions are controlled by you. In terms of the strength of the Trickery domain, it's a strong domain in PnP.
Also the problem with saying you could interchange Cleric, Paladin, and Warlock is bad.
If they allow multi-classing of companion NPCs, Larian will be implicitly saying it is in fact not bad.
warlocks generally do not serve gods for the most part
A warlock's patron is whatever it is in the world the GM has created. In Wyll's case it's a naked blue-skinned aerobics instructor with horns, wings and a tail. But it certainly could be a deity.
while Paladins and Clerics do, so that's one important distinction...Wyll could not be a Cleric or Paladin and still have the same patron or for his story to still make sense. And then Shadowheart beyond obviously not being able to be a warlock could not he a Paladin because there is no Paladin of Shar subclass in the game...wouldn't work.
There is no "paladin of Deity X" subclass in D&D, either. Paladin subclasses are determined by their Oath, not what otherwordly entity they adhere to. Cleric subclasses are not even determined by the entity the character adheres to. That's determined by the Domain. Shadowheart could just as easily be a Death domain cleric (rules as written, the Death Domain is an NPC-only Domain, but she is an NPC).
You speak about the game as if it has an existence independent of the person playing it.
