It's not even completely necessary to know what to expect and do any prep. It's doable without foreknowledge, although still pretty difficult.
PCs are more powerful than basically what encounter builders say, typically. I've found that a "deadly" encounter is typically only deadly in cases of really bad luck or if the party has already used a significant amount of their daily resources.
With consumables, backstab/highground, and/or the ease of resting in BG3, I'd up any party's effective level by ~2-4 for any official 5e encounter builder.
The PCs aren't "more powerful" per se, but the ability to jump huge distances, push as a free action, disengage with zero risk, light swords on fire with candles, use scrolls from any school of magic with any class, backstab as any class, hide in plain sight as any class, and carry multiple full sized barrels of oil around, have definitely screwed up any semblance of balance when it comes to trying to figure out what is an adequate challenge rating in this game.
