Generally speaking, it may be the case, but not always.

Case in point: 1996. BYU had a running back by committee, with Brian McKenzie (Jr), Ronney Jenkins (Fr), and Mark Atuaia (Sr) all having more than 80 carries that season (the next closest rusher had 45. Their stat lines are below:

Atuaia: 86 carries, 365 yards, 4.2 yards per attempt
Jenkins: 128 carries, 733 yards, 5.7 ypa
McKenzie:167 carries, 950 yards, 5.7 ypa

Atuaia was at least serviceable. Both McKenzie and Jenkins had 1,000 yard seasons in following years (1997 and 1998, respectively), though both had a significantly higher workloads and reduced YPA as a result of being the feature back (McKenzie, because Jenkins officially took a medical redshirt in 97, but was actually suspended; Jenkins, because McKenzie had graduated). Arguably, both were good to very good, with Jenkins potentially being great, but each were somewhat hampered in their BYU legacy. McKenzie had injury issues prior to his senior year, and was only a two-year transfer, so he is not often remembered among BYU great RBs. Jenkins is often cited as a great "what-if" who left early due to honor code issues but still had a serviceable NFL career.

That neither had a 1,000 yard season during one of BYU's all-time seasons does not mean that neither were good, and like I said, it can be argued that having both of them led to increased effectiveness in terms of YPA for both, since it went down for both in the absence of the other. Of course, there are other factors at play there, but it's at least debatable that having a running back committee does not inherently mean none of the running backs are good.