I have an old world seed and even then I'm not sure if I've ever felt like not having found a single core in a dungeon. Probably cleared about ten for 20 cores now. It's basically not any different from burial chambers and surtling cores, though I think burial chambers are guaranteed to have a room for cores since shortly after launch. Not sure if that's the case for mines, but I think it could be.

The identification of infested mines is also more or less exactly same as burial chambers. On my world I still keep running into chamber entrances which are half-buried or obstructed by trees. Skeleton signalling helps, and I'd compare that to the wisp lights and the overall more pronounced exterior structures of mines.

The terrain and limited visibility is an obstacle, but I've lately come to think that it's offset by density of resources in addition to telegraphed enemies (obviously). Compared to plains, you can walk quite a distance until you end up on a plains-unique resource that is not cloudberry or deathsquito. And even in plains, you can't always see the squitos behind birches or in heavy mist. Many players tend to be taken by surprise by growths too, apparently.

Mistlands is very densely populated by giant remains, dvergr, organic harvestables and so on which are all quite valuable or useful for variety of purposes. The reward makes up for the danger, but it can take a while to get to that point where it feels fun, and there's nothing exceptional about that.