I personally think Flake is being underestimated. The margin in Arizona was undoubtedly much closer in 2016 than in 2008/2012, but:
(a) For all the talk about how close Arizona was in 2016, Hillary only beat Obama's % by one percent.
(b) Trump was undoubtedly a worse fit for Arizona than McCain/Romney, given his problems with college-educated whites, Latinos, and Mormons. But Flake wouldn't have similar problems with these groups, particularly Mormons.
It's harder to tie Flake to Trump, given Flake was one of Trump's highest profiled critics in the GOP.
This guy underperformed Obama by 5 points and has been often polled as one of the most unpopular senators in the country.
I'm not saying he's doomed, but seriously, he's definitely not underrated.