Shame on me for not recognizing that, I tested it and results were clear: on non-desert map it took 5 min for Sabaen levy spearmen to beat indian spearmen unit whereas in desert map it only took 3 min, so I guess they shouldn't be viewed as ultimate underdogs.

On the part of Carthage being victorious on every game: quite probable considering that they have resources to fund strong armies and powerful navy, SPQR players would have quite the challenge.