An interesting article in the Wargames Illustrated issue of February 2020, concerned gamming with war weary warriors. In reality, the psychological effects of combat are finally being understood with PTSD.
Putting this into a game context shows how morale can affect a units performance. Sure most rules have some sort of mechanism to note this, but a formation being, inexperienced or veteran can both face the same issue.
For example, if a formation has been say fighting a campaign, and has suffered severe casualties, or prolonged exposure to sustained artillery will be none too keen to continue to stay in the line or obey orders.
Just another kink in the cogs of gamming to consider.