the Scipii have less terrritory to exploit to the south and east than the others and therefore will have a tougher time than the other Romans when the CW breaks out
Which is why I made the suggestion to have an eye for that eventuality by putting expansion into Africa on hold, landing in Greece, thereby stifling the robust economy the Brutii would otherwise enjoy

It's the worst, easily the worst in the game. What units will man their siege towers in the time of war? Hillmen? Couldn't even beat basic Hastati on the walls
Agreed. A Parthian campaign has to depend on merc infantry, adding to the economic woes.

As for Dacia, the reason I would keep them in the nearly impossible category is the fact that all your neighbors have a superior unit roster to you and you are going broke
Which is why it's imperative to eliminate Thrace ASAP, and take Scythia's best city, Campus Scythii. In my Dacian campaigns, the biggest threat is always Macedon. The AI loves to build scads of Light Lancers and countering them early on is not easy. Barbarian Cavalry is adequate (barely), but you need to bait the Lancers into the AI's preferred wedge attack, and hit them on the flanks. Falxmen are lethal to militia hoplites and other lower tier infantry, but once you get to Chosen Swords, they become the mainstay of your battle line. Like Germania, Dacia gets Chosen Archers, which, unlike other foot archers (excepting Gauls Forester's) can engage in melee combat.

I've never found Dacia to be overly difficult except in dealing with Macedon's cavalry, so we'll have to agree to disagree there

However, I still think its hard because you need to clean the Mid-East, Anatolia, and Egypt out quickly in order to avoid facing the Romans when they are far more powerful
In a players hands, the Seleucid roster crushes anything (even post-Marian) the Romans can muster up, IMHO.

I'm better with Cavalry
And like I suggested, this influences your list as far as difficulty is concerned. 4 of your top 7 get excellent cavalry