They have more hp and more damage in age3. Even if the landskns are weaker in age 2, highlanders don’t get any upgrades in age 3, except the arsenal ones. Even so, in the average supremacy game you’ll be unlikely to have advanced arsenal so landskns are actually better stat for stat, aside from the range business.
As I said, landskns are still very much a threat in age4, whereas highlanders are made obsolete by 2 musketeers. Highlanders probably need a niche and changes more than the others.
I don’t think either Landsknecht or Highlanders get any upgrades in Age 3, aside from arsenal and civ specific cases. In any case, Landsknecht has 345 HP, which is lower than the 400 HP of Highlander.
Considering only the basic Arsenal, Landsknecht can buff their HP to 379.5 (345 * 1.1) due to Infantry Breastplate. On the other hand, Highlander increase their attack to 38.4 (32 * 1.2) due to Socket Bayonet.
This means that, in a melee-only fight, Landsknecht can only take 15 hits (14.1 hits = (1.1 * 345) / (1-0.3) / (32 * 1.2)) from Highlanders, while Highlander can take 16 hits (15.5 hits = (400) / (1-0.4) / (43)) from Landsknecht.
So Landsknecht loses to Highlander 1-on-1 in a melee only fight… That is problematic.
This actually a big issue. Generally, ranged infantry units are better than melee infantry units, as they can be micro’ed, avoids snared, can shoot all at once without pathing issues, etc. To compensate, melee infantry generally needs to be faster or have more HP, as they have to survive to engage in melee. The fact that Landsknecht only moves at 4 speed, has lower HP, cannot shoot, and costs more than Highlander thus means that either:
Either case can be justified, but the current situation is not balanced.
Highlanders are a bigger threat in Age 4 compared to Landsknecht, as the Highlander can counter most ranged cavalry very effectively, while Landsknecht cannot. In any case, if Highlanders are made obsolete by 2 Guard Musketeers, then does not Landsknecht get obsoleted by 2 Guard Halberdiers?
I think that is more of a Sweden problem, than a Landsknecht problem, as their merc buff is a bit extreme.
He said as if that wouldn’t already make the Landsknecht obsolete in comparison
So all I’ve learnt in this thread is that a guy with a gun beats a guy with a sword - big surprise.
Casual Highlander Landsknecht interaction 
Stop complaining about Mercs.
They are strong Age3 and fall off after Age4 for most civs besides Germany and Sweden. This is fine. Highlanders are not OP you have some serious fixation issues if you believe that they are.