why do you think that would be harder? assume for the moment that the reader here is going to run at the effective rate of a single node and not that we're going to try to parallelize that. Assuming we have transaction isolation, that reader is going to get a consistent snapshot.

a distributed database is potentially more complicated to operate, and optimize, and because its new and potentially has more sharp edges maybe less reliable (?) but the extraction of moderate sized datasets doesn't really seem to be an obvious failing.