Historical German army experiences stress that the need for discipline has its roots in the extraordinary demands of combat itself.
The German keyword here is Gefechtsdisziplin - combat discipline. It's the compound of obedience with thinking and comradeship.
A (small) unit cannot withstand the stress of battle without discipline, thus discipline needs to become natural for army soldiers. It needs to be trained with discipline in little everyday affairs, but the superiors should always remember that it's combat, not the everyday affair that warrants this effort!
This is of utmost importance, for exaggerations that do not pursue the goal of robustness under combat stress will stifle the "thinking" part that's of great importance for actual performance in battle (and for developing leaders).
As a consequence, it's quite unimportant whether all soldiers wear the sleeves up, down or whether they mix it. They may march in lock-step or not.
All that counts is that superiors used enough discipline standards to instil and maintain discipline. Discipline is a skill that need training and maintenance, it is not a performance.
Trust is another almost all-important ingredient for robust combat units. Cohesion enhancement measures such as esprit du corps, regional recruitment or common food for all ranks are one path towards building mutual trust. To lead by example is another important one.
Trust is an important defence against wavering under stress, and it's the primary bonding agent of units. It's furthermore of great importance for low level independent action and for the reduction of friction.
Combat discipline and mutual trust are the basics of robustness in combat.
Gucci gear personal equipment, modded rifles, expensive tanks, big budgets and even combat experience are no substitutes and all built on feet of clay if discipline and mutual trust are gone missing.
To prove a force's superiority over an (obviously) lesser opponent does not in itself prove that the job of building and maintaining discipline and trust was done well.
Only crisis in face of local opposing forces' superiority reveals a force's basic soundness.
"Combat proven" or not - we better pay attention to the basics. Tools, toys and numbers already get more attention than necessary.