As SS2 said, Shiki's main power to rival Roger was facing his one ship with an entire fleet, with Shiki only losing due to an unexpected storm damaging his fleet and sticking that steering wheel in his head. He was still a rival, having well against both Garp and Sengoku, as well as being recounted as one of the top tiers of the era by Sengoku during the Whitebeard War, though he never received as much attention as Whitebeard. It's also very telling that Sengoku knows Shiki is alive, yet still referred to Whitebeard as the world's strongest man.