Against skirms it is always better to use them in melee. Skirms have 30% range resistance and Oromos’ peak ROF is only on par with melee. Best case scenario you can do 70% of their melee damage and worst case scenario at maximum range (like when they’re retreating), they’ll be doing 26.25% of their melee damage. Skirm’s damage output against them is the same in melee or range but since they kill skirms quicker in melee, they’ll take less damage overall. If you’re microing them you’re not going to lose 30% of your damage output to bad pathing.
The problem with the unit is that the answer to if they should be in melee is basically always yes. Against heavy infantry is the only time they should use their range, but even then they’re not great due to a x0.5 multiplier.
As they are now, the unit just doesn’t work as intended. It also doesn’t serve the heavy cavalry role that Ethiopia needs. To make it somewhat viable it needs stats something like this: