Sanders clearly has a better shot than anyone else to win, but a seven in 10 shot is not an overwhelming favorite. It means that there’s a decent chance Sanders won’t win.
The lack of confidence we should have in the Nevada outcome is partially because of the lack of polling data, but also because the polling data has not been particularly predictive in the past.
Since 2008, Nevada has been a polling wasteland. Looking at all candidates who polled at 10% or better after undecideds were allocated, Nevada polls taken after the Iowa caucuses have had an average error per candidate of 8 points. The 95% confidence interval for each candidate above 10% is something closer to +/- 20 points. That is, to put it mildly, a huge range.
The polling for the 2008 Republican caucuses was particularly bad. Mitt Romney was favored to win by 5 to 15 points. He won by nearly 40 points.