Not being able to carry as much as a human means a lot for soldiers.
If you can only carry 40% of the weight that means the standard equipment of the US Marine would be a nightmare to an elf.
Mithril equipment can balance this but far from being an equal situation (it just ensures elves don't break their wrist for holding up guns that weight 4-5 or even more kilos, because they can't).
Admittedly they also use human soldiers who benefit from the mithril equipment but the lightweight metal is a bane for projectile mechanics. Lighter bullet means they penetrate less. It can be solved with composites but adding stuff would only make the manufacture more complicated which is an issue for smaller caliber ammo made in huge quantities.

The fragility of elven bones also means they're almost always dead when crashing with a plane. Survival rate can be rather low.
Being slender and less massive also quite a considerable tradeoff during melee or brawls which unlike your expectations is something which does occur a lot in combat. Not being able to take a decent hit to the countless wrong places is also an issue in this aspect.
Same for taking bullet, blast or fragment wounds. As I said, elves are fragile. Though they are way too proud to admit their weakness.

I can detail it further but I really, you were ought to connect the dots yourself.