Robin Waterfield is the translator of the version of Polybius' Histories I have in hand. I have no other versions for comparison, and I certainly don't have the Greek original to check, so my opinion of this translation counts for little. And yet I make two observations:
1) This is very "easy reading" for ancient history. Part of the credit for that belongs to Polybius, who knows how to narrate. And part of the credit goes to Waterfield, who has done a good job of bridging the language gap. I remember reading in the preface something about his decision, for example, to break long sentences into shorter multiple sentences. Modern readers, after all, can't hold their breath at all as they wait for the period.
2) Waterfield's efforts to speak in modern idomatic English also result in some troubling choices for a work of ancient history. For example, in one place Polybius is complaining about the way Fabius (another historian) has been less than completely reliable:
In my opinion, however, readers can safely assume that Fabius is reliable more often than not, but they should not regard what he says as gospel; rather, they should base their conclusions on the facts themselves.Uhm, did Polybius, writing in 150 B.C. tell his readers not to regard what Fabius writes "as gospel"? Okay, that communicates in English, but it is confusing historically.
Polybius (2010-11-11). The Histories (Oxford World's Classics) (p. 138). Oxford University Press. Kindle Edition.