I'm not sure "suffers" is the right word here. Radiant soul applies to radiant or fire spells and celestial warlock adds sacred flame automatically. Later it adds wall of fire. Truestrike is already and option and so is hellish rebuke. Pact of the tome can easily add more cantrips to which it might apply.
Really? Because I wouldn't call any of those worthwhile.
Sacred flame (and indeed any other cantrip) is worthless in comparison to
eldritch blast--you get +5 one single time ("you can add your Charisma modifier to one Radiant or Fire damage roll of that spell against one of its targets."), instead of +5 on every single hit. And since EB is force damage, there's literally never a reason
not to use it over
sacred flame or any other cantrip, including
fire bolt.
Wall of fire is particularly bad, because it would literally only get you a single instance of +5 damage. If it were once per round, then persistent spells might have some value, but as it stands, the feature is nearly worthless in the vast majority of cases; by the time you get it (6th level), everything else you can do is simply
better.
If the design goal was mainly a bonus to cantrip damage and sacred flame is always available it looks like a it's got a guaranteed attack cantrip to which it applies if a person wants for free up eldritch blast for something like minor illusion instead.
It's not so much that radiant soul suffers if it's meant to mainly be used with cantrips. It's that eldritch blast is so good because of how WotC treats the attacks that every other damage cantrip largely suffers in comparison for warlocks.
Okay. I don't base arguments on what I think the class should have been. I base it on what is actually there. And what's actually there is that this effect is, mostly, worthless even for celestial warlocks
trying to use radiant damage. Getting--in effect--+5 damage one time per slot and one time per cantrip cast is pretty terrible. Even the Evoker Wizard's 6th level feature (which is also about dealing damage; your cantrips become "save for half" rather than "save and no damage") is almost always superior despite coming at the same level.
If it's meant to be used mainly with cantrips, it should be made with
the signature, special Warlock cantrip in mind. Hell, I wouldn't even mind if the feature simply said, "When you cast
eldritch blast, it deals radiant damage instead of force, and can be affected by this feature." It'd still be only +5 damage per casting, but at least it would play nice with the existing feature.