Baktria are a bit more challengeing, they have a great unit roster which also includes alot of "odd" units, they don't have Elite phalanxes tho. "downside" is that they have quite annoying neighbors - the AS and the Saka rauka, the Parthians could be annoying but they seldom are.

The reason for Ptolemies being considered easier(than the Seleucids) is quite simple: they have less and less dangerous enemies, that do not directly threaten their coreland. The Arche has to cope with Hay, Pahlava, Balkh, Ptolemies and pontos from the beginning and Koinon Hellenon, saba and Saka rauka once they get past initial problems. the Ptolies actually have direct acces to the Seleucid core lands, in Syria and mesopotamia. The only initial enemy of the Ptolemies is the Empire, who don't threaten egypt but the levant and Asia minor. Later on conflicts with Saba and Karchedon are likely but the Carthagians are far away and the Sabeans, well they have skirmishers.
And about the roster, it's somewhat a toned down AS roster, apart from galatians there is not much new to find but you initial possibilities are just as good. The AoR of most units is worse than their regular counterparts, which really does hurt expansion. The Pharaos may not be able to train Cataphracts, but at least they get the Hellenic heavy lancer which many people prefer over actual cataphracts and the semi hellenic light lancer which is only availible there and in Illyria.