People who think the Rams undervalue development simply don't understand NFL team structure. These so-called experts who think that and say that kind of thing haven't a clue that their blanket statement makes no sense.
At the top of the hierarchy is McVay. His role is to be an innovator, and creator of the team culture all the while managing the team overall and dealing with diva egos both coaching and players. Sean is not a teaching coach, he expects his position coaches to do that part of the workload. A good head coach must by necessity delegate the workload, breaking that work down into individual units, i.e. RBs, o-line, WRs, TE. QBs, d-line, LBs, secondary, STs, etc., etc. Each unit is overseen by a position coach with his own subordinate assistant coaches. It is these coaches that develop the talent. How well that happens depends upon how good the coaches are.
For example, in the McVay era, they have had excellent coaching for the defensive secondary. Look at how well players like Rapp, Fuller, Johnson, and now Scott have played. To say there is a lack of development there would be completely wrong. Contrast that with the o-line who as a unit has struggled to develop young talent under Kromer.
QB is much the same in so much as we have witnessed how Goff's development was stunted by poor coaching, both from his position coach and especially from McVay who completely sabotaged Jared's development with his micro-management of Goff's play. I've been saying this for a couple of years now. Sean won't do that to Stafford. It is why I simply don't see them drafting their "franchise" QB as long as McVay is HC. They would rather trade for a hired gun like Stafford.
RBs, and WRs have progressed as expected which reflects solid coaching for those units. So you simply can't isolate deficiencies of some players and ignore the blossoming of others on the same team.
Yes, there have been problems with the o-line and QB and those two have been addressed. The o-line by kicking Kromer upstairs and replacing him with Carberry someone with a proven track record of developing college talent. Kromer looked fine when he didn't have to coach up young players and the line was comprised of proven vets. When the line got younger, Kromer's shortcomings were exposed. The issue with the QB was solved by simply trading for Stafford who doesn't need a position coach and with whom McVay doesn't feel the need to micro-manage his play.
When players don't meet expectations, it could be coaching, but it could be a lack of talent in the player, his work ethic, or perhaps a poor fit for the scheme. Troy Hill is a perfect example of that. Under Phillips' man-coverage preference Hill was miscast. It was clear that Hill was a zone CB, not a man coverage CB. So when Staley installed his zone-heavy secondary scheme Hill's play improved. Darious Williams is a perfect example of a talented and motivated player being taught proper boundary CB techniques. Darious Williams is a perfect example of player development by good coaching.
So the answer to the question is an emphatic no. They do develop players, but some position coaches are better than others at that particular skill.