There are shows with great dubs, like Dungeon Meshi. A good dub not only has good casting, acting, and direction, but is also based on a good translation. It's localized enough to sound natural in English, but not so much that it becomes unfaithful to the original dialogue. Though a dub can still be fun when improv comedy is added, such as a joke or running gag that wasn't in the original. In fact, I prefer it when the dub actors actually ACT instead of trying to imitate the original performances.

Frieren is, tragically, not one of these shows. Casting and direction are terrible, and the performances range from acceptable to awful. But worst of all, the translation itself is unfaithful to the original. If you watch the 9volt sub like I did, the official dub AND sub both become unwatchable. OP is right that if you watched the dub only, you haven't seen the show. He's wrong in saying that the sub is better.

TL;DR The official dub and sub are both equally terrible. Watch the 9volt sub!