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.
