Spoiler free response: Maybe.

I love the game (now over 1500 hours). Some points to consider:

-You'll need to start over with a new world. No problem for me, but that has some upset. If you try to go to the Mistlands on an old map you will likely end up in a messed up biome. I believe this is where some of the negative feedback is coming from... the "issues" with the biome are magnified because it's not generated properly on the old map.

-It's hard. Remember that jump in difficulty going into the plains? Yup. More so. I'm by no means an elite player and I find it doable... just a tad too hard to really enjoy.

-You may die ... a lot. Some of the "too hard" comments come from the fact that people who rarely die are dropping like flies (even hard-core players) and getting upset at the skill loss penalty (or gear getting lost in a spot they can't retrieve it). New enemies have a knock-back that's almost absurd.

-It's different. Someone said something like "the swamp and the mountains had a baby." I think it's interesting (albeit hard) and actually like it better than the Plains.

-Player progression is heavily gated in the Mistlands. It's going to be quite some work to unlock the new weapons and armor. Could be an issue if jumping through hoops or grinding isn't your thing.

Overall I like it ... I would enjoy it more if it was tweaked a bit, but certainly enough to keep playing.

If you're in Valheim for a more casual building and exploration experience and are risk-adverse, you may want to pass on the Mistlands. If you left Valheim because you'd mastered every mob and just didn't think it was challenging any more ... I say "gear up"!