They're not, but shadowheart is already a cleric and you only need one.
All the domains are cool, but none compare to life domain - all the healing spells you would want come auto prepared, every memorized slot can be offensive and CC spells, and lets face it, no one is going to use turn undead on plants or whatever, having divinity charges for a mass heal is 1 charge = 1 short rest.
Clerics *are* boring. I've been around a gaming table enough times to hear the eventual sigh, followed with "OK, I guess I'll play the Cleric." Warlock is the third most popular choice and the party already has a Warlock, too, so the argument that we don't need another Cleric is undercut by the argument that we don't need another Warlock, but Warlock is a popular choice and Cleric is not. When I say it's boring, I don't mean it's boring for everyone, just that it is boring for the majority of players. Cleric = Armor, Mace, Shield, Healing Spells. Take away armor for some builds (for something else) and make Healing part of the class abilities like I proposed above, leaving spell slots open for more interesting things, and the class becomes more interesting to more people.
People aren't making Warlocks because of anything to do with Wyll, firstly they want a CHA main, secondly a lot of players are making multiclass 2 Warlock / rest Sorc or Paladin. This gets mixed in with Paladin and Sorc statistics if they picked Warlock at their first level I would guess.
A lot of players are viewing Warlocks as one of weakest classes due to the lack of spell slots, but they want 2 levels for the Eldritch Blast plus its two upgrades.
I took EB on my sorc with the spell critical feat, but without the warlock upgrades its pretty pointless unless enemies are resistant to other cantrips. One complaint about this thats already got a thread here is not being able to use the warlock spell slots before the sorc slots, making them quite useless as they likely wont be used up before you short rest, and cant be used to fuel metamagics, this is why I'm avoiding the build or any multiclassing for now until I reach level 12, then I will play around with testing whatever builds I can think off using Withers and use multiclassing in my second playthrough.
Regarding Clerics being boring, yes this is true in most other games and maybe PnP, but not the case in BG3. Their offensive spells are very powerful and useful, and you can see Shadowheart being used as a staple companion in just about every youtube playthrough of the game. Even without changing her domain because a bunch of players have found lots of neat tricks with abusing Polymorph which she gets with her default trickery domain.
