Siege Ram Question

Can someone explain something thats always confused me that everyone says about siege ram. Ie, what is the power of the siege ram over the capped ram? Both work fine vs archers or buildings. A little more hp? I don't get it. Whereas every other upgrade like say Arbalest or Siege Onager or Heavy Scorpion are HUGE improvements in obvious ways that almost make their previous versions unusable in Imperial Age. And why do people say certain civs have no answer to Siege ram like the britons? Do they also mean no answer to Capped Ram? Do Briton halbs really struggle to kill a ram? That hasn't been my experience in late games.