So, with only two rolls (to hit, defend) there's no doubt, the OPR system has less room for delineating strengths....it can be more vulnerable to fluke rolls. If you're used to more complex and competitive rule systems, it's going to be an adjustment.
One of my favourite games is ADLG, and that's just a roll off system lol. Sometimes one side gets a +1 here and there for being a superior unit.
I see several ways to rationalize the mechanisms of melee.
1. About the fact that if the defender survives or is not demoralized he can act normally (he is not locked). It is then necessary to imagine that the charge of the attacker was a failure in the sense that it did not break the opposing lines and that the defender managed to keep his positions and his cold blood while pushing the opponent . Without necessarily making losses but at least keeping it at bay. There are other things that dices do not tell.
2. About the fact that the unit that charged is vulnerable to fire after melee. Already the shooters must have a line of sight. Except following the configuration the unit that has defended blocks the lines of sight (difficult to explain without drawing). So there is a strategic element in the placement of the units. If I take into account the first point then it is not absurd that allied units nearby help each other by firing at the enemy unit once it has been pushed back.