Isaac Asimov talked about how in SciFi it is important that while you're in the future, you create a logical and consistent world. Otherwise it doesn't work.
The problem with this book is it is illogical but any common sensical measure, even with today's world.
First off, the enlisted men in the Legion are the best fighters ever, not just individually but as a unit. But they are given officers from the point who are political nincompoops. That doesn't work. If the officers are incompetent and political, then that pushes down into the enlisted ranks. This is why the militaries in dictatorships are so bad. Yes you can have an occasional officer who has made it so far, but not a widespread practice.
Second, in the first battle they have 35% killed & wounded. They are now combat ineffective. Unless the mission was strategically so important it was worth the complete loss of the unit, and in this case it was not (it was just show the flag), the unit would turn around and immediately head back to base.
Third, when they are asked to go attack another tribe, even a political officer at the rank of captain would never do that. Nor would the other officers agree. That is a diplomatic decision and would be pushed up to the commander of the forces on that planet.
Fourth, they send 2 soldiers to track the alien vehicles. But they have reconnaissance drones which they do use appropriately later. What - the competent captain suddenly forgot about SOP in this case?