Nobody has time to watch every single prospect in the draft on an individual level. You find scouts that you can trust and then rely on their analysis when forming an opinion on a player.
I've compiled the CBS rankings, so we can compare how the prospects were rated versus when the Patriots selected them, and a quick fact from PFF. The unsourced quotes are from PFF press releases.
#61 Alabama CB Cyrus Jones (CBS Rank: 122): "Plays the run aggressively. Comes up hard and fast against run plays or screens to his side and will fight for position rather than just occupy space and turn it back inside. Had a top-10 run defense grade and the highest coverage grade of those ten corners."
#78 N.C. State OG Joe Thuney (CBS Rank: 157): "One of the nation's top guards in 2014, Thuney made a smooth transition to left tackle in 2015 finishing fourth in the draft class at +35.6 overall. He surrendered only seven pressures on 507 attempts in pass protection and at the very least he'll bring versatility to an offensive line as a capable guard or tackle."
#91 N.C. State QB Jacoby Brissett (CBS Rank: 213): "Brissett's arm is as strong as his frame. When he drives the ball to the sideline, it gets there in a hurry. The arm is also a big help for Brissett on downfield targets, where he's fully capable of hitting receivers in stride. Consistency, though, remains an issue."
#96 Nebraska DT Vincent Valentine (CBS Rank: 281): "Valentine is strong at the point of attack and stout against the run (+21.4 on 466 snaps in two years) but he provides little as a pass rusher."
#112 Georgia WR Malcolm Mitchell (CBS Rank: 125): "Uses his body well to shield defenders from the ball. Caught 19 of the 20 catchable passes thrown his way on hitch routes in 2015, picking up 242 yards in the process."
#208 Eastern Illinois LB/S Kamu Grugier-Hill (CBS Rank: 312): "Grugier-Hill lined up at ILB in the three games we have graded from 2014-2015, and made 12 total defensive stops." (note: KGH comes from a small school and all of his games aren't graded)
#214 Houston LB Elandon Roberts (CBS Rank: 233): "Roberts lined up at ILB, and his +19.7 overall grade ranked 21st of 402 NCAA linebackers in 2015. He earned positive grades for run defense, pass rushing and pass coverage."
#221 Illinois OG Ted Karras (CBS Rank: 263): "Ted Karras had the second-best pass-blocking efficiency (98.4)among Big Ten guards in the 2016 draft class."
#225 Arizona State WR Devin Lucien (CBS Rank: 435): "Lucien may have the best hands in the draft. He dropped just five passes over the past two seasons, despite QB play that made most receptions a challenge. Even "accurate" passes were all over the place from a ball-location standpoint, forcing Lucien to make impressive catches on relatively routine receptions."
UDFA Auburn CB Jonathan Jones (CBS Rank: 153): "Patriots UDFA Jonathan Jones had the third-best tackling efficiency (10.1) among SEC CBs in the NFL Draft."
UDFA East Carolina TE Bryce Williams (CBS Rank: 198): Williams was the 4th highest graded offensive skill player in the Senior Bowl, including a tie for the 3rd best receiving score.
UDFA Ole Miss TE De'Runnya Wilson (CBS Rank: 306): "QBs targeting Patriots UDFA De'Runnya Wilson on vertical routes (corner, post & go) had a 132.8 QB rating, 5th-best among WRs in this class."
UDFA Arizona State WR D.J. Foster (CBS Rank: 309): "Part running back, part wide receiver, Foster can be used to create matchups in the passing game where he forced 13 missed tackles as a receiver in 2015, fourth-best in the class."
UDFA Florida Atlantic CB Cre'Von LeBlanc (CBS Rank: 463): "After Week 2, Patriots UDFA Cre'Von LeBlanc only allowed 1 deep pass completion (20+ yards in air)."
UDFA Vanderbilt TE Steven Scheu (CBS Rank: 489): "In 2014 Patriots UDFA TE Steven Scheu averaged 1.60 yards per route run, it would have been 10th-best in this class if he'd repeated it."
UDFA Mississippi State LB C.J. Johnson (CBS Rank: 530): "Patriots UDFA C.J. Johnson had the eighth-best grade against Power-5 teams from this LB class at +15.8."
UDFA Texas A&M FS Devonta Burns (CBS Rank: 930): Literally nothing about Burns available. He was a nickel corner in 2014, but was relegated to special teams in 2015.
When asked for comment, PFF added: "Burns played all of 17 snaps last season which might've been why he got missed by the PFF_College account.
The year before last he played mostly slot corner (only one snap at FS) and his yards per cover snap allowed Sig Stat from the slot in 2014 was 0.76 which was 37th in the nation and 14th among corners who were subsequently part of the 2016 draft class.
That number in 2015 would've been tied for 16th in the class."
UDFA Ole Miss DT Woodrow Hamilton (CBS Rank: N/R): "Patriots UDFA Woodrow Hamilton earned a +12.7 run defense grade, 8th-best among SEC interior defenders in this class."