As for Van Gorder, I'm 100% convinced that there are some scheme issues. I'm not a smart enough of a football guy to really break down those issues, so I'll let a smarter person opine on that like I've seen on ISD, etc. They exist, and they generally have to do with the variability/complexity getting in the way of the players being in the best position to maximize their strengths.
WITH THAT BEING SAID... the biggest issues are personnel driven. So I don't know if I blame BVG for not molding his scheme better to the players he has, or if I blame Diaco/Cooks for just a laughably pathetic job of recruiting from '12 onward in their areas of responsibility. The cupboard was left really fucking bare, and BVG did everything imaginable to add 1) emergency fillers in the '14 class 2) tons of players we haven't really yet gotten a chance to see in the '15 class.
Remember, Diaco won with nearly 100% Weis recruits + a star player he signed in '11. Whereas offense has built depth and talent every year since Kelly has been here, it's impossible to look at the defensive recruiting and think the same. They landed a couple ELITE players but left insane depth holes at other positions.
I'm not sure how we can look at a roster of Jaylon Smith, Cole Luke, KVR, Redfield, Shumate, Sheldon Day, Jarron Jones, Isaac Rochell, Nyles Morgan, Drue Tranquill, Greer Martini, Andrew Trumbetti, Romeo Okwara, Nick Watkins etc. and say there isn't any talent. Those players are talented. Its the coaches job to get the best out of his players and put them in a position to be successful. I highly doubt that at any point going forward we have a meaningful upgrade in talent on defense overall.
The highest rated player in the two cycles of 2015 and 16' classes, as of now, is Shaun Crawford at a composite of .9456. From 2012 onward, so 2012, 13', and 14' ND signed Jaylon Smith, Max Redfield, Nyles Morgan, Isaac Rochell, Elijah Shumate, and Sheldon Day, all rated higher. Lets basically call it a rating of .94+ rating. So in 3 years Diaco signed 6 and in 2 years, BVG will have signed 1, hopefully more, but he'll have to sign 3 (not sure if that happens) in order to get up to the avg. of 2 per year Diaco was at during this very poor recruiting effort he had. What would I rather have 2 per year or .5 per year?
A majority of the contributing players on this defense were recruited from 2012 onward. In fact a majority of that list, in the first paragraph, is Diaco players that are plenty capable of playing high level football.
To the depth point, that isn't a Diaco problem as much as it is a ND problem. Because ND doesn't oversign or kick underachieving players out we have to have a higher hit rate. We've lost a lot more players due to injury and early NFL draft on Defense than offense during this time period. Thats just the way it is, Diaco didnt make those rules and BVG is having to deal with them too, thats why we failed miserably last cycle to hit on the right number of true S targets. How many DE's did BVG bring in last cycle, 1, Bo Wallace who never made it to campus. 2 positions where we need more depth.
To the last bolded, that proves he's a good coach. He can take a group of players, get them to buy in, and be successful even though they may not have been perfect for the system or "his" guys. Diaco took talent and made it better. He had a group of lost and confused players that had never seen a successful system before. A lot of players spoke or have spoken about that since. Some that were playing different positions for the first time. I don't see too many players on this 10-3 team that BVG can say that about. If BVG had taken over that roster, I don't even want to know what would've happened or the excuses we'd have to come up with.