In terms of competition, there wasn't anything or anyone to stop Roger from getting the One Piece aside from his illness killing him sooner than expected. The battle with the Rox happened way before he went on his final voyage for the OP, Garp and Sengoku didn't bother him unless he was messing around with other pirates, Rocks himself is implied to have a different ambition, and so did Whitebeard (wanting to have a family), Shiki (ruling the world, not being PK), Kaido, and Linlin. Even after the existence of the OP was revealed, only Kaido and Linlin were the ones from the old generation (that we know of) who expressed interest in getting it. Getting the OP is integral to achieving Pirate King status, and Roger was the only one going for the treasure in his time, as far as we know.
This isn't to say Roger didn't have a tough time, but compared to Luffy's journey, he had a smoother voyage.