The Mistlands are a challenge, but I didn't find my second and later excursions into them any more of a ramp up in difficulty than the Swamps, Mountains or Plains were. In fact I found Plains more challenging, although with the reduced visibility Mistlands can result in a lot more "surprise" encounters.

When I first stumbled into the Mistlands I didn't even have the Wisp Light, a huge mistake. With the endless fog and the challenging vertical terrain it totally sucked.

Later I came back with maxed out gear and the wisp light, and on exploring more I did find flat areas suitable for farming and for easier travel, but there is admittedly still a lot of vertical terrain which in some ways is worse than the mountains. I died multiple times but

I've figured out a few things that have vastly improved my experience and survival rate in the Mistlands...

1) Don't go in without maxed out gear, your best food and the wisp light. And don't get caught there at night. Really, Valheim has always punished you for going into new biomes unprepared so this shouldn't be a secret. I really don't think it's worse than the previous biomes in terms of difficulty, I think most of the people gripping about how horrible Mistlands are have forgotten the pain they experienced when first exploring the Swamps, Mountains or Plains biomes. I still hate Fulings and Deathsquitos more than I do Seekers and Gjalls.

2) The Dvergr are your friends... until you make them otherwise. If you find a Dvergr tower or mine, mark it on the map and use it as a refuge, if Gjall or Seekers are chasing you, getting to a Dvergr settlement will mean you have several of your little buddies beating down the monsters in short order. Only take out a Dvergr base with a lot of prep, planning and caution, ideally only attack one that has already been beaten down badly by monster attacks. And only take them out to get the bits you need for sap extractors.

3) Build a base in an adjacent biome or in an easy to defend location on the coast of the Mistlands. My current Mistlands base is a tower on a long, very craggy peninsula. It's far enough out in the water that it is mist free most of the time and gets few spawns other than hares. The walk down the rocky terrain of the peninsula is terrible, but a few hundred wood later I had installed a causeway and bridge system that takes me right into the mainland with little effort. Doing this on frequently used routes is a bit more work than raising paths in the swamp, and the same as building stairs and mountain paths so it is doable. And Ygdrassil wood and wisps are not hard to collect, so scattering those around will make for an easier to find and more comfortable run out or back.

4) Seekers and Gjall will destroy your crops. So don;t hang around your planted areas except when it is time to harvest and replant. Fields of Jotunpuffs, a chicken farm and bee hives are all you need for the Honey Glazed chicken meal, which is currently one of the best haelth foods in the game, so make lots. Misthare supreme is a little better, but you can't breed Misthares so availability and time spent to hunt the clever little bas---ds is a downside. Eggs from the chicken farm and Jotunpuffs will make Mushroom omelettes, the second bet stamina food in the game. You won't need many Magecap mushrooms until you get into magical weaponry, so concentrate your mistlands farming on Jotunpuffs initially.

I could add more but this will already be a TLDR post for many readers. So don't give up, you can beat the Mistlands just like you did every other biome.