I can either be too vague or too clear. Too vague svcks (obv) cause you don't know what your own character says, which ruins RP. And too clear ruins RP too, because, to use DA2 for exemple, say you want to romance someone. Having the <3 icon svcks because you're not picking what your character will say to seduce them (though in DA2, "seduce" is more like "you're dark and broody, I like dark, rawr") out of a list of unmarked lines, you're just picking the preset romance path for the dev's character essentially.
But imo what's most threatening to RP immersion is just having VAs in itself, not because I want mutes (though I do prefer that personally), but because actors use their own interpretations to pick a tone when they record lines. For exemple, the male VA said he saw "Mr Howard" as a family man (pretty sure he said that anyway, citation needed, etc), but what if you don't see your dude PC that way ? Or if (and this is purely hypothetical) the female VA tones her lines to invoke feminity and you want to play a tomboy ?
Though on the other hand, the ME and DA voice actors had interpretations too presumably and they were ok for the most part, so it might be totes fine too. My point is, there's a lot you can [censored] up when doing voiced PCs, you can do it really good and be fine, but you can [censored] up because it adds complexity to the development. You can't [censored] up a silent protagonist. Well you can if your writing svcks.