Youth structures go through cycles and you only have to look at the history of youth football to see that.
It needs good recruitment (scouting), good development (skill and tactical coaching), good competition (internally from other players and from external league and cup competitions), and a clear pathway to first team opportunities (a first team manager who will give the kids a chance)
On top of that personal motivation, parental support (realistic expectation management), good facilities and one or two other things probably come into play.
I don't think anyone disagrees that we have fallen behind Manchester City in this regard (during a time of stability at City and instability at United)...but I'm not sure we are in crisis like some people suggest and we are not behind other clubs. I certainly don't see a plethora of Academy players breaking through elsewhere.
We set high standards and always have done, so expectations are higher at United than elsewhere. We are going through one of the cycles like in the late 1970's, mid 1980's, and probably mid 2000's.
However, we are still producing first team players which is the point. You can argue all day long whether you like them or think they are good enough. They are playing.
Every decade in our youth history (from 1932) we have brought through 6-7 players that have had very good to excellent careers at United. For most of the time they have played their part in helping us win trophies. That has continued up until the present day.
Unfortunately the fact that Manchester City and Liverpool (largely without a strong youth contingent), have been the most successful club in terms of trophies over the last 4-5 years and we have won nothing (cycles again) means their is an increased focus on what we are doing. That focus leads to increased criticism.
We have been investing again in recent years (after a lack of investment when Woodward arrived) and it takes time to produce new talent. We have reached the U/15 Floodlit Cup Final, the U/16 Premier Cup Final, the finals of the National Qualifiers, topped our group in the UEFA Youth League and were very unlucky to be eliminated, and obviously the final of the FA Youth Cup for the first time in 11 years.
Finally, we do need to improve in certain areas, particularly scouting in my opinion, and others have mentioned coaching so let's see how well we deal with those challenges.