Jaguars are much better than Conquistadors, Ballista Elephants, and Gallic Swordsmen.
Conquistadors get their big bonus against melee and since they've been moved back in the tech tree from Guilds, they usually will be facing gunpowder units anyway. I guess they're great against the HRE, compared to regular Curaissers but their bonuses are a lot more fluff than substance.
Ballista Elephants are barely an improvement over regular elephants - it's not like it's hard to kill mounted units anyway, and they are resource dependent. You still need axes, swords or maces to deal with those pesky spears and pikes.
Guerrilla I isn't as good as Woodman I, and the fact that Jags are cheaper and don't require resources makes them great for rushing. Gallic Warriors are great if you have copper but not iron, but so are Jags, and if you have no metals, Jags are the best all around melee unit in the game. (Though Dog Soldiers beat them heads up, but then rushing Sitting Bull early isn't one if the brighter game plans.)
With Jags, you can take over an enemy city, and cheaply turn their population into your troops. It doesn't matter what resources are hooked up - you don't even need a trade route to your capitol, and Jags are pretty cheap. Jags are uniquely suited for a snowballing slavery powered rush.
I don't think much of the Phalanx myself either. Possibly because I generally prefer to attack with swords and catapults. Let's face it, in BTS the Greek UU's special power is that it's the same as a pre-Warlords axeman. Which makes it a small improvement over axes, but I have a hard time getting excited about them.