From my own vantage point, I agree very much with the warnings. It is most unwise to underestimate any Republican attack machine. And although late in coming, the Trump television ad campaign which has started is a sign the fall election will not necessarily be a romp for Mrs. Clinton. The article attached here gives some examples which I won't repeat now.
Suffice it to say that supporters of Hillary Clinton will indeed need to stay very engaged with wallets open in order to secure the Presidency.
Clinton Campaign Warns Against Complacency