Exdeath accomplished alot yeah, and similar to what Kefka did. However Exeaths accomplishment wasnt nearly as devastating to the people of the planet(s) as what Kefka did. Exdeath merged the worlds, blahblah, but Kefka flat out ravaged his. Not only did Kefka succeed in rearranging the face of the planet and killing countless people in the process, he continued to destroy towns and people for the fun of it, all while you struggled to reorganize your troops. The damage Kefta did to the planet would also never be completely fixed once he was killed. You could rebuild, but the planet would always be changed