My suggestion about the mining dangers: make them rare. That way, the threat is still present, so you might want to be prepared. The problem with wraiths isn't that they are particularly dangerous, it's that they are annoying. I for one, don't want to deal with them every ten minutes or so - I've killed around a dozen of the things during initial fort setup, I think that's way too much.
Balrogs are fine: rare and powerful, but beatable, somewhat like the way that clowns are beatable. The soggoth however was kind of a unfair, I always hated creatures that were only possible to kill with cave-ins and the like. Sure it takes a lot to deal with a frost giant siege head-on, but it's possible. Even forgotten beasts made of steel are possible to kill with military might... Judging from their description alone, at first I thought soggoths could be killed if all of them are dead at the same time, and if I kill the source before the offspring springs to life, they stay dead. So I threw everybody who could hold a weapon against them.
Granted, it was a bloodbath. If I could somehow get the soldiers out of there and keep the soggoths in, it would be a new variation of the danger room: every soldier received a massive stat increase by the time they collapsed, and even after that, the soggoths couldn't penetrate their armor. Sadly, it's just 20 unconscious dwarfs in a walled-off room, with something like 150 soggoths hacking at them constantly. I think they'll starve to death before they die of injuries.
I think this is mostly due to their mithril armor however. Anything weaker, and they would be dead already.
Another idea, if possible: make the soggoths dependable on food. If there are no dwarfs or any other food available, make them die after a couple of months. If this is possible at all, of course.