A 2004 paper by Joe Shipman, then director of election polling for SurveyUSA, showed that polling on ballot measures had triple the rate of error (9.5 average error on the margin) as polls in presidential elections (3.4) and nearly double that of contests for statewide offices (4.6). I summarized the assumed reasons for that greater error rate in a long post four years ago today, but the most relevant to Maine are a greater difficulty modeling the likely electorate and the problem of accurately conveying ballot language.
Silver is more positive about the Washington referendum. I'd say: look at the age of the voters in exit polls. An off-year election tends to attract the hardcore, and they tend to be older, which is why fears about Maine are well-founded.