If you were to get equal english counterparts to the seiyuu, just the western dubbing process can take some of the synergy out.
The Japanese record their lines together,they are all next to each other hearing the lines being delivered and able to change their line to compensate. Example:
Dude a: Oh hey dude b. Late again to class huh?( said with an dissapointed downwards
inflection)
dude b might give his reasons slowly with a little shame in his voice, as they both know he's always late.
The english VO could be just as talented, but he's reading lines off paper by himself with no way to get the tone of voice or other subtleties in the script. He doesn't know if dude a was understanding yet dissapointed ( then dude b would follow the japanese seiyuu route) or if he was more condemning and strict, in which case he'd say the lines faster and more defensively.
Stuff like this.
And the fact you must match the sounds to the mouth flaps and you lose out on the slang and honorifics, like calling the teacher "Yukari-chan" instead of 'Yukari-sensei'.
An english version would have the character maybe say 'Yukari' instead of 'Ms Yukari' but that's not the same, as -chan is a suffix for mainly young children, also not saying any suffix can imply a close relation or disrespect. This is also lost.
Dubs ALMOST NEVER translate the damn kanji yet they show it on the screen like we're supposed to read the handwritten letter the main character is silently reading ourselves? Add to the fact if you know japanese some then it's VERY IRRITATING to say the least that the english accents mostly ruin the names.
Example. (from Clannad)NuhGEEsuh.
Did the character say: Negisa, Nagisa(it's this), nogisa?
the terrible Inuyasha dub: KuhGOOUmei
Kagoumei? Kagome(this)? KEgome? Kegoumei? Kogome? Really guys?
also the R sound in dubs is usually iffy at best and wrong mostly.
I still like dubs for trigun, bebop, and VO Luci Christian, Chris Patton, and Johnny Bosch to name a few.