but we still haven't seen their ceiling, from the thieving cat or from the masters, as they couldn't go nuke on the fight as they could injure Cross, and I say nuke because they have nuked places already hahaha
we do see the Trio effectively erase a city off the map, give or take, in a flashback when they first came across the lightning elemental girl (Sylphie, I think her name was) - it's just a panel, but they were up against a veritable army with a fortified city, and it was narrated as "gone" in the aftermath.
So yes, "nuke" is accurate in terms of rough equivalency - city-leveling, at minimum. But I also think it's not worth talking about their hypothetical ceiling, because past a certain point they could potentially disrupt things like continental tectonics or the magnetosphere of the planet which would spell an apocalypse for life on the planet this all takes place on.
And when you can do that as an individual, the concept loses all meaning because you can't show that sort of power in a meaningful way - so it becomes useless in conversation.
For Seras compared to that - Thieves aren't trying to destroy continents. I would say that a fair comparison is that there's no vault or secret that can't be accessed or uncovered by her, however much a person or organization or nation state might try.
So she could "erase" a city from the map by stealing intelligence and leaking it to that city's enemies, thereby weakening it to the point it can be crippled and toppled by opposing forces that could only achieve that by way of Seras' abilities.
But that's a level of world-cohesion granularity that might go beyond what the author themself has considered when putting all of this together, so.