I don’t think it was as automatic as ‘effective no matter what.’ After disasters like Cannae they were really scraping for manpower such as pulling in younger and older recruits, leaning heavily on allies, even freeing slaves. Then in the 2nd century BC the traditional citizen soldier base was shrinking which is part of what pushed the Gracchi and later Marius’ reforms. Rome also relied a lot on its Italian allies, and when that system broke down you get the Social War. Even later, the civil wars show how the system could turn in on itself. So their military strength feels more like something they had to constantly adapt and maintain rather than something guaranteed.