To be clear, I'm not saying hoplites were unable to operate effectively in uneven terrain. Obviously there's a lot of evidence that they were very capable of operating effectively in all terrains. But they weren't designed for that, and it certainly opened up vulnerabilities that were particularly noticable without skilled commanders leading them (and later successor armies were probably more open to these vulnerabilities than the phalanx of Phillip and Alexander). And regardless, they were definitely not more effective at it than Roman armies.
Also re: the pike phalanx, the pike phalanx of Alexander and Phillip were both less rigid than those of their successors and more importantly less central to an Alexandrian army. The battles of Phillip and Alexander were more often than not not decided by the phalanx (which also had a large supporting contingent of lighter troops and also more flexible troops), but by the cavalry. That was the particular advantage they had over the other Greek states.
No ancient militia was ever 'designed' for anything; the Samnite fighting style organically arose from a combination of factors. Insofar as terrain was a factor in its emergence, basically identical terrain played a similarly indeterminate role in the emergence of the Classical hoplite phalanx. Nor is the role of terrain in the Roman adoption of this fighting style at all clear. Moreover, the advantages we're discussing relate almost entirely to
attacking in broken terrain; the hoplite and pike phalanxes were both fine at
defending hilly positions, which is what's under discussion here.
The advantages of defensive lines in mountains [as opposed to rivers] are the scarcity of provisions in sparsely populated areas and the physical effort taken to cross them [especially with small corps occupying the passes, however they are armed or trained]. The disadvantages are the difficulty of lateral communications from pass to pass and the lack of population to support a large army.
River lines offer good lateral communications and logistical bases for stronger forces than mountainous areas can directly support, but they're much less arduous for enemies to cross.
As an aside, I also don't think it's been proved that army composition changed all that much under the Successors, or that what change their was meaningfully hindered them; it's certainly asserted a lot, but the pike phalanx was only a portion of the armies of e.g. Antiochos III at Magnesia [a minority in this case] or Pyrrhos. At Magnesia Antiochos had no fewer than 12,000 horse, providing an even greater superiority over the Romans than he enjoyed in foot; he broke the back of Egyptian power at Panium using his cataphract cavalry.
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