Perhaps this will help : A short faction breakdown.
Starting with
Spain :
The beginning can be extremely difficult, depending on how you plan on doing the first X turns. Your units are all inferior to all enemies, and Gaul and Julii will relentlessly send armies to kick your ass. The only trade partners could be Britain and Numidia.
Marius Reform Romans are very, very, VERY bad as Spain. You have one unit that can stand your ground. Late game is hell.
Unit wise : Scutarii are OK for the first X years. But get annihilated by any elite unit. Cavalry is terrible. BW are a very strong, but very expensive and very late unit.
Gaul : The beginning is tricky, but after a few well played turns the game gets rather easy.
Marius Reform Romans are very, very, VERY bad.
They have all sorts of mediocre infantry late in the game, amazing archers, decent cavalry.
Thrace : Depending on how many enemies will dogpile on you early on, it can be very tough to pull out the first few years.
Marius Reform Romans are very, VERY bad. (only 2 "very")
Unit-wise Thrace has a poor roster for the late game. But i would say their phalanx, even though pathetic, can give an enormous edge against Romans. Wall of pikes + Bastarnae flank charge and one CAN dominate Marian Legions.
Thrace is the only faction on the list that i have played SP before. Not too long, but i did.
Carthage/Seleucid : First XX years are probably the most difficult in the game, enemies throw entire stacks at you.
But once you survive it, these are amazingly strong factions that roflstomp enemies (like Armenia).
Very good units across the board.
Also : If anyone ever again calls Numidia "Hard", i'll exile you (again!)