Paladin spellcasting is potent and also unique. A cleric/fighter does not just flatly surpass any paladin. They can self-buff their relevant stats, twice thanks to their unique spells, and get access to tools like Holy Sword and Sacred Haven, the latter of which is a unique party-wide +2 AC that stacks in all reasonable cases.
Cleric does beat out paladin when it comes to spells. Especially war domain for fighting, healing domain for healing, and the spells in general before then.
The only thing a cleric can't do is cast Aura of Glory and will have one less attack per round, on the bad end of attacks. The sheer power up from spells outpaces Smite Evil's limited uses. Divine Power + Bull's Strength stacks up their strength twice, domain powers and Aura of Vitality access can do it a third and fourth time.
Holy Sword is good, but a cleric can cast greater dispel from a distance. Sacred Haven is good, but clerics have Undeaths' Eternal Foe which does more.
Divine Favor, Bless, Aid, Prayer, Battletide, Divine Power, Greater Magic Weapon, Lion's Roar... it adds up to more AB than a paladin can get on their own. Even with the nerf to divine favor, that's a +19 spell AB bonus, it almost hits the cap of +20, and thats not mentioning all the spells a cleric has access to, or the conversion of excess spells into healing and considerably more spells than a paladin in general.
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Though all that is irrelevant, clerics are better, gods give too much power, people misuse it etc etc.
If you play a paladin I would hope it's not because you want to play a strong character, there's ample enough other builds for power, and there's a lot more to a paladin than that...
... and there's only so many changes you can make to D&D until it's no longer the version of D&D that Ravenloft has been supporting & adhering to.
Paladin's have heavy multi-class restrictions here, but it's not too far from the source material.
If a paladin stops levelling paladin in the source, they'd still keep their paladin powers until they break their oaths, but they can never again advance as a paladin as it requires a constant heart.
Due to the nature of things like the relevel crystal and when forced relevels need to be done due to the introductions of new skills and feats, the source material restrictions doesn't reconciliate well with the ability to rebuild a character from the ground up. Acquiring more total levels means a potential to rebuild for more paladin levels before multiclassing again, with no time efficient way to verify it, so this is likely why the stance on multiclassing paladin here is the way it is.
Its more based on logic to the systems that are currently in use than any particular jab at paladins that want to multiclass in general, and all classes here get twice as many feats as usual so paladin already has access to far more customization than they normally do get.