Bad VA can take me right out of the moment.
Modding Oblivion atm (which circle of Hell am I in?) and when I went to try out Better Cities, this was what I immediately noticed. Some of the voice acting for mods seems so terrible that they have to be doing it on purpose. Unless you have good VAs and audio production quality which is at least 90% up to Bethesda's standards, modders should really just stick with non-voiced dialogue.
I did notice they put together some kind of guild of amateur voice actors with acceptable levels of VA talent/skill. I don't know how successful it is. You're probably expected to throw a few bucks their way when you hire a few which is okay, I guess, considering the time they put into it. I remember one modder hiring the actual voice actress that did the VA for Veronica in FNV for his mod which I thought was kind of cool
It's become much better in recent years, and you now see big mods like Vigilant having what I would call an "acceptable" level of voice acting across the board. At this point the biggest problem with voice acting in large, popular Skyrim mods seems to be that no one can pull off a convincing Nordic accent.