I haven't found the Mistlands to be that bad really. It reminds me very much of the swamp. By the time you have found enough crypts to make all the upgrades, you don't really need to be in the swamp anymore. And they swapped out the constant wet/cold debuff with mist. Personally, I spent more time looking for swamps that had a decent number of crypts and fuling totems than I did infested mines.

You can lift the mist as you go by simply building wisp torches. There is enough Yggdrasil wood and wisps are so plentiful you can pretty much just burn it away as you go. Also, if you get to the valleys that have no mist, hit a tree with your axe. If you see the stealth eye go wide, wait for the enemies to come to you.

I do agree about the uneven terrain. Since Valheim is just poor with swinging over and under enemies when elevation differs, it seems strange to double down on that. Regardless, I wound up just using the bow most of the time, which negates that draw back. If you use Drauger Fang and frost arrows, you can kill a standard seeker in three arrows, maybe two plus it will be slowed. The new bow is actually better since Spinal Snap top loads all the damage as pierce. Yes, poison does more damage, but since the seeker fights are all relatively close quarters, you will typically shoot another arrow before the poison DOT finishes, meaning that damage is just flat out lost.

Mines, yeah, they tend to be few and far between. Although in the last larger mistlands area I was in, I found three mines, although they were spaced a ways apart. But there were three or so giant skulls to discover in-between. The four mines I've cleared produced 2,2,8,3 dark cores. So that search ended with 4 mines cleared which certainly isn't awful. I am at only 8 of 9 sealbreaker fragments, so I will have to go back and try to find a few more.

I didn't have an issue with having essentially beaten the mistlands before I could make any gear, because most other zones function that way too. The problem in my mind, is that the mistlands follows a zone that doesn't work that way, the plains. In the plains, all you need to make the padded armor is iron and some thread. So just steal some flax from a fuling camp, grow it on an isolated plot and you have the armor the entire time you are clearing villages looking for totems. And then you are still stuck wearing it for most of the mistlands adventure, which sort of gives this sense of stalled progression. Versus finally having all the forge upgrades and enough iron to make the iron gear and then immediately finding silver.