Theoretically I don't think there is actually a limit on which factions have reforms or how many, but there is a limit on the number of units (500 or 499 for M2TW I believe). This factor actually limits the "need" of reforms the way I see it. Additionally, historically speaking, most, actually if not all of the end game factions were completely destroyed or assimilated into Rome. Also remember history is written by the victors (hence why a rather detailed history of carhtage is impossible). There is a wealth of information regarding the romans and hellenes, but less regarding the more barbaric factions thus it would be improper to inroduce reforms and units based heavily on guess, speculation or imagination.