izigame.me

I got this for the two new nations, so I'll focus on those:

~~Piedmont~~

A very balanced nation with a mix of bonuses including great Priests, cheaper 18th century pikemen techs, and a unique 18th century Dragoon that's second only to Dutch Dragoons in prowess. Seems to have a lot of minor differences rather than a few big ones.
That may not be the most exciting selection of bonuses and unique units, but sticking to a more generic roster does make Piedmont flexible. Specialized nations such as Algeria or Prussia might dominate in certain aspects of the game, and their clear-cut game plans are often tempting to newer players, but they usually pay for it by being weak in others. Force one of those nations into a situation where their strengths no longer apply and they fall apart. By contrast, "generic" nations like Piedmont can do pretty much everything at least competently, meaning they can tailor their strategy to whatever the needs of the game are.
Also, as an Italian history buff, it's just cool to see Piedmont (AKA the County of Savoy and later the Kingdom of Sardinia) in the game at all.

~~Switzerland~~

Here we go. Switzerland boasts several unique units, two of which are straight upgrades to already common units. First are their 17th century Pikeman that's second only to Spain's Coselete and Scotland's Sword Clansman among 17th century melee infantry. They're more expensive and train slower than most other pikemen, but they kick lots of ass and give the Swiss a formidable early game punch. Mind your defenses if you spawn close to a Swiss player!
Then in the 18th century the Swiss get two more unique units. First are the Jaegers; musketeers with high damage output and very long range, but low HP and no melee attack. They also train slowly so I don't recommend them as a replacement for your standard Musketeers. They're more intended for skirmishing and harassing the foe, poking them from outside their range and avoiding any direct retaliation. I'd say they're an optional unit, but having options is always good.
By contrast, the Mounted Jaeger swaps their gun for a sword, and replaces the Hussar as Switzerland's 18th century light cavalry unit. It's a very simple unit - just a stronger, more expensive Hussar. In fact, not counting Poland's Winged Hussars (which are more like fast heavy cavalry), it's the strongest light cavalry unit in the game!
I'm not sure why Switzerland of all nations gets such a strong light cavalry unit (granted, I know jack squat about early modern Swiss history) but Mounted Jaegers are a good unit. Probably your go-to late game cavalry.
The tradeoff for all this awesomeness is that advancing to the 18th century is more expensive for the Swiss and their 18th century Barracks build more slowly. Given how long those normally take to build, this is actually a bit painful and means they'll be slower to getting key late-game infantry compared to other nations.
These drawbacks combined their elite Pikemen reinforces Switzerland as a nation that's strongest in the early game. Be aggressive early on, then advance while your opponents are still recovering from the beatings you dished out. Your late-game armies of Musketeers and Mounted Jaegers (plus more Pikemen) are perfectly serviceable; the trick is giving yourself the breathing room to transition to them.
Overall Piedmont and Switzerland are welcome additions to the game's roster. Add in all the other features you get and the Golden Age is probably worth its base price, and absolutely worth it if you get it on sale.