They're a less extreme version of light infantry.
They have less accuracy etc than rifles (still more than line) but worse melee than line (still more than light/rifles), while having the reduced ironflesh and powerstrike of rifles/light infantry.
They're just a medium between light and line really. They don't really specialise in any one field, which, to me, makes them a bit rubbish. Give me proper light infantry over Grenzers any day.
Regular lights are just riflemen with muskets in essence, where as Grenzers are less accurate than riflemen but with either bayonets or rifles.
If you give them rifles, they are not useful, as they aren't as accurate as riflemen but still being vulnerable to cav and (given how most rifles/light units train) useless in melee. When you're fighting in melee, the difference in damage is much less relevant than actual ability to use the weapon (rifles in this case), so the difference in melee between rifles and Grenzers with rifles is negligible, but at range rifles are superior. If you can use rifles well in melee, Grenzers still won't make a huge difference compared to riflemen, so there's very little gain.
In essence, I'd never use them with rifles equipped (just use riflemen instead
).
Compare them to regular light infantry and the same thing happens, with light infantry (of other nations obviously) out-shooting them but there being a negligible difference in melee damage.
That being said, they out-shoot line-infantry and have only a slight disadvantage in melee compared to lines, but the main advantage is actually the formations more than anything. In other nations, light infantry out-shoot lines by more than in Austria, so the advantage of having lights in Austria is less than other nations.
TL;DR: Lights are useful, but Austria's light infantry is nerfed compared to other nations.
