TL;DR: The unit is far too expensive for the Swiss or its allies to produce, which actively discourages their use.  If you haven't used it yet and are curious, read my whole rambling post for analysis.  For my thoughts on how to fix this, jump to the end with "Redesigning the Reisläufer".

Era IV has several culture-specific Halberdier replacements, and you'd better believe the Swiss have got one.  The base Halberdier is an excellent Era IV unit, the last great melee unit before gunpowder units start to dominate the battlefield. The Halberdier appears at the start of the era's tech tree and immediately outclasses Era III units, is known for a generally lower production cost than other units you'll make at the time, but with a high population cost.  It's also a solid unit upgrade, especially for any "citizen armies" you may have raised as a Militarist culture in your past.

How does the Swiss unit stack up against the other halberdiers?

Not well.  Let's compare the base unit and the cultural alternatives.  Benefits in blue, downsides in red.

• Base Halberdier: CS41 (Cost 485industy 3pop)
• Japanese Samurai: CS46 (+5 above base), +1 Move, cannot retreat (Cost 485industry, 3pop)
• Sapa Inca Guards: CS43 (+2 above base), higher CS on good terrain (CS49) (Cost 485industry, 3pop)
• Swiss Reisläufer: CS43 (+2 above base), higher CS while defending (number unknown) (Cost 1945industry*, 2pop)

All three emblematic units are stronger than the already-good base unit.  The Samurai is significantly better all the time, while the Inca and Swiss units can outperform even the Samurai in specific but common situations.  I currently don't know what CS bonus the Reisläufer gets from "Defender".  I'd assume a +3 to +6 which would give it a CS46-49 while defending.

Where the Reisläufer falls down is its eye-watering 4x production cost of its counterparts.  This unit is so expensive that it's prohibitively slow to build an army or to upgrade your existing units.  Its unique trait, "Mercenary Corps", makes it less expensive to build when you have more Arms Deals, but even with five arms deals, the Reisläufer still costs double the other empire's halberdiers.  If I had an Arms Deal with the Swiss and it prevented me from building my own Halberdiers, I would cancel that agreement.  On the plus side, the unit is 1pop cheaper than the others, and that's actually significant for Halberdiers; unfortunately, it doesn't balance out how terrible the production cost is.  The current design of the Reisläufer means that it comes with a massive weakness, and it has a special mechanic that only partially mitigates that weakness.

Redesigning the Reisläufer 

Let's see about making this unit a true mercenary army, giving a unique, flavorful benefit that encourages Arms Deals and also encourages their use:

• Lower the production cost to 485industry again to make this a unit that we can produce, and which your partners will want to produce
• Remove the -10% discount per arms deal; we're getting our "mercenary" benefit in the last item on this list, below
• Probably keep the pop cost at 2 to make them competitive alternatives to Halberdiers
• For balance reasons, we probably have to tweak the base CS and whatever CS we get from "Defensive".  The plan is that this will be a more common unit, compelling from its cheaper population cost, but also a unit that most empires can have access to. The Swiss will want to sign Arms Deals, because...
• Finally, and the unique benefit: Make it so that a chunk of the unit upkeep costs for foreign Reisläufer is paid to the Swiss as incoming money each turn. The more mercenaries the Swiss provide to other empires, the more they get paid!  This could be 50%, 75%, or even 100% of non-Swiss owned Reislaufer upkeep going into the Swiss coffers

Thanks!