Is think you nailed it with your assessment of later picks in the first round. They usually end up in the 20-30 range because either they were sliding in their draft year; or in the scouts opinion, there were areas of their game like size, skating or hockey sense that created question marks.
And it seems more often than not, they simply plateau and top out as journeyman or AHL-NHL 'tweeners'. But if you do your homework you can hit 'gold' in the back half of the first round. You mention Cory Schneider. But Ryan Kesler and Brock Boeser are also living proof of that.
And the Bruins are more than happy with David Pastrnak, who was drafted in 2014 at #25, one pick after the Canucks picked Jared McCann.