Well it depends what way round you view it:
Reaction 1 is reversible so establishes equilibrium = disadvantage.
Reaction 2 is irreversible and therefore goes to completion = advantage.
From the second point of the mark scheme I think it's talking about it from the 2nd perspective because reaction 2 is the dangerous one, not reaction 1.
(Which actually doesn't fit the way the question is worded. Mmm idk why the mark scheme is phrased like that, but if you're really clear in your answer about what things you say are an advantage and what things are a disadvantage I'd hope you'd still get the marks.)