Danrama
Archetype
I believe my beta readers have been honest, but I doubt they've told the whole truth. It's a difficult task, because every reader makes a subjective judgement and what works for one may not work for another. I think the beta reader should tell you their main problems without nit picking. It helps if they are writers themselves and not your nearest and dearest.
I try and stress to my beta readers that they are Beta readers and not editors. I find it hard when I get sentence corrections and even had one pull out her red pen (avoid teacher types) before handing it back. So I understand what you say when you mention nitpicking. I want pace/plot/character and world-building points primarily.
In the end my editor always seems to be the most objective and useful point of information on all these notes. Yet in saying that I have had some invaluable input from two specific readers but one has had a baby and the other is far too busy lately for my second novel... leaving me betaless!