At full XP (it goes up to 9) units are much, much tougher than the standard version. As far as I have been able to tell every experience chevron gives stat boosts to accuracy and reloading and morale, and possibly to melee and defense as well. Maybe the fact there is no difference is a bug? I'm sure my units all got better with more chevrons.
Do some tests on a custom battle to really see the full effect of the experience bonus.
Also experienced units seems to fire a more well-timed volley, and they also perform certain firing drills more professionally than raw recruits.
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