@vahnj A corollary of the second point: if you are under attack by barbarians who keep re-scouting you, more tech Will Not Help because the next barbarian spawn will use it while you still have to build or buy defensive units.
Another one: your military tech advances in the ancient era put substantial pressure on every civ in the game even without any saber-rattling from you, because it makes their barbarians worse, not just yours. But don't study anything you aren't prepared to fight.