They're higher on average, but you lose out on control (especially compared with point buy). Rolling all 13s is, in theory, higher than the 27-point build allows, but given that most characters have only a couple of important stats, such a character would underperform.
(Of course, a lot of people "roll randomly", but keep rerolling until they get a set of stats they like, and if not allowed to do so will suicide a character they don't consider to be good enough. Which effectively gives them the best of both worlds - higher stats and good scope for optimization. But the rules can't really help you with that one.

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