Half the Ultramarine legion died on Calth, one of their most important worlds, and even more were left stranded and killed off during the underground war. Even more Ultramarines were massacred when their number one most important supply nexus and recruiting world, Armatura, was also destroyed by the World Eaters and World Bearers. Then they were led on a wild goose chase that ended with them running all they way Macragge and turtling for years after demon Angron almost killed Guilliman.

Ultramarines spend a lot of the early heresy taking big strategic losses and the middle part of the heresy doing very little other than rebuilding because of Lorgar honestly. They didn't beat Lorgar, they survived him.