Back in the Oblivion and Fallout 3 days these were really amateurish productions. I guess it has a lot to do with the availability of good and willing voice actors and equipment.
For Oblivion, Vilja was the standout. Vilja set the standard for all subsequent mod companions. Never really used companions for Fallout 3 until mod author Kris Takahashi developed some characters in anticipation of Fallout 4. That was the first introduction for two of the companions for his Tales from the Commonwealth mod.
Skyrim is where mod companions hit its zenith. Bethesda learned something from how well mod companions were received by players. Unfortunately now in the age of microtransactions, great mod companions are a relic of the past.