I'm probably in the minority who think trading for Lehner was a perfectly fine risk to take with a 21st overall pick. The only difference between risking that pick on a prospect and risking it on Lehner, is that Lehenr is NHL ready. It is the same type of risk, since the Sabres will have control over Lehner as an RFA, and if anything players are much easier to project at Lehner's age than they are at 18. Look at the Senators scouting record in the first and second round since 2008. They pretty much don't miss on those high picks. Tim Murray played a part in that.
If you're expecting Lehner to come in and be a .915 goalie behind the worst place NHL team last season, or if you're insane enough to expect him to come in and be an elite goalie, you'll be hugely disappointed. He's 23 years old. There were probably other goalies who would have been a bit better right away, but the difference is that Lehner has potential to be better than those options a few years down the line and he can grow with the Sabres.
There are some huge red flags with him. Lots of speculation about poor attitude, him being a headcase, etc.
In 13-14 he had a .913 save percentage in 36 games behind a Senators team where everything was going wrong. Prior to that, he had 17 games of .930 hockey over two seasons. Because of this, there was a contingent of Senators fans who were SUPER high on him up until mid way through the 14-15 season. If you went back in time and told them he'd be traded before the 2015-2016 season they'd jump off a bridge.