I played through act 1 and it was good. Stopped only because I couldn't decide whether to stick with Polish or English voice acting. Took so long to decide I dropped the game because of it :P
I saw some contextual stuff or swear words, or even senteces completely left out in the english dub, wrong tenses or different words, all in the tutorial area. And as the english subs are made for the english VA, I didn't really want to stick with english and miss out on this. I can understand polish somewhat, but had to replay some conversations several times, look up some words etc. so it wasn't really a fluent experience. Kept bouncing around and when I finally wanted to make the final decision at the end of act 1, I never got back to it.
I've found some odd mentions about this on ancient forums around the web, most importantly this:
https://rpgwatch.com/forum/threads/extended-english-subtitles-in-witcher-link.3147/
But the link to the alternate subs there no longer works.
Also tried Czech VA and it was better in some and worse in other regards than English. Still not optimal/missing a ton of stuff. And I would not be able to stick with it throughout the whole series as there is no Czech VA in Witcher 3.
EDIT: Going through this thread got me to fire the game up again. Probably finally found my golden combination. Polish voices with czech subtitles. Already found several occurences of missing/different info mentioned in the subs, but I can usually get that from the polish voice. The czech subtitles (to me) have the advantage of usually having the translation of words I don't understand in polish so I can somewhat keep up in such cases without having to replay every conversation two or three times, which I had to do when I tried with polish voices and subtitles. The advantage of czech and polish being somewhat similar :P
But either way, the game would greatly benefit from fan made subtitles which would contain everything that is actually being said. Kinda surprised I seem to be the only one bothered by this. I've never seen anyone complain about it. But I guess it's pretty easy to see for me due to being Czech.