They are the most accurate free predictor available, because they use real retired questions, the real adaptive engine and College Board's own per-form conversion tables. They still drift, mainly through test conditions and through form selection, and three of the four known sources of drift inflate the score rather than lowering it.
The asymmetry is what matters for planning. Three of the four known sources of drift push the practice score up rather than down, so a Bluebook result is better treated as an optimistic estimate than a neutral one. Plan against the lower end of the range it suggests.
The full picture, with the surrounding detail, is in Is Your Bluebook Score Accurate? What Practice Scores Predict.