I am not seeing the role of smite spells other than providing a modicum of utility. They take an action to cast as well as a bonus action on a hit. This translates into eating at least 2 attacks at level 5 or above plus an additional attack on hit if dual wielding. Given the way DRS works in this game, a single attack is worth a lot more than it is in 5e. I can't see any case where casting a smite spell could make up for the damage lost due to forgoing attacks, especially after 5th level. With damage stacking, a paladin could get a stronger single divine smite when combined with thunderous smite. However, using divine smite on critical only, the expected value of this combination would be limited and not match the damage lost by forgoing attacks.

Am I missing a mechanic that would otherwise make smite spells viable from a damage perspective?

Do they work on ranged attacks?

Searing Smite can potentially do some damage, if you maintain the concentration and have the target burn for multiple turns...

But in general Smite spells are there for utility, hence why most of them provide an additional non-damaging effect (Thunderous provides knockback/prone, Wrathful provides Frightened, Branding prevents invisibility and Blinding provides Blind)

Divine Smite is the pure damage use of Paladin spell slots. Not only is it additional to any attack (Rather than costing a Standard + Bonus action like Smite spells), but also provides just raw damage. Every other spell slot usage is just pure utility (Including any multiclassing into spell casters, it will be hard to find anything more destructive than multiple attacks with Divine Smites attached unless you're just dipping Paladin and can access high level AoE spells)

I'm not sure how Searing Smite fares if you build for it with some sort of 2 Paladin/10 Fire Draconic Sorcerer - I'm not sure how its damage scales with upcasting and Dragon Sorc's CHR mod on spells (If it affects every tick it could theoretically do a lot of damage... But if that's worth over just slapping something with Divine Smites as a regular Paladin build is another matter...)

What is the airspeed velocity of an unladen swallow?

I am not sure. In 5e, I think they are linked to melee attacks, but given the wording of the spells, I suspect mechanically they could work with ranged attacks in BG3. That being said, there are limits to the value of ranged attacks on a paladin given the competing opportunity of divine smite.

Right, I asked because I was wondering if ranged attacks would be an edge case in favor of smite spells. In general you are better off just regular smiting I think, but sometimes you cannot make a melee attack, and regular smites don't work on ranged attacks.

Damn ... thats right.
How did i missed that. O_o

You are aware that you don't have to choose between smite spells and divine smite? If you have set divine smites to "ask" then every time you hit a target with a smite spell you will be asked if you want to add a divine smite on top.
This certainly burns your spell slots, but allows you to deal a ton of damage quickly.
It is especially powerful when combined with an item that ensures a critical hit. Then you can deal 14d8 radiant damage (+ whatever damage your weapon does) with a single hit.

The issue becomes that smite spells cost a standard and bonus action. Meaning it would be more beneficial to simply utilize both actions for normal attacks allowing you to add Divine Smite to both attacks.

Given the plethora of ways to attack with a bonus action (Dual wielding, Polearm Master, Great Weapon Master and Duelist's Prerogative) it becomes exceedingly less worth it to use smite spells if damage is your main concern.

With the sole exception being maybe Searing Smite because of it's potential 10d6 of burn damage.

This is further exacerbated if you care about maximizing spell slot usage so only smite on a crit, whereby not only does normal attacking save you the cost of the smite spell, but also means your bonus action attack is an additional crit potential.

African or European Swallow? I watched MONTY PYTHON as well.