Well, without some sort of limitation in place: Move to the enemy group, Fireball, Action Surge, Fireball, Misty Step away...
You certainly can do that with 2 levels of fighter but you've burned 3 spell slots on one encounter. In addition, you'd have to wait till 7th level at the earliest when other full casters were tossing fireballs at level 5. In PnP where long rests matter, that's a significant loss of resources. But in BG3 currently, why not? Just rest up and repeat.
Yeah, that's the main problem currently. The rest system in its current state means that every single spell/ability is practically per-combat, if needed. But there is now a whole thread regarding the rest system, so let's not discuss it here.
Although to be fair, it could kind of be discussed here too, since it's clearly not meant to be that way in DnD 5th edition. There are recommendations for how many short rests and long rests one should use in campaigns and such, and naturally it shouldn't be possible to constantly spam long rest. It's absurd. They did that far better in Solasta too, where there are certain places on the map where you can long rest.