Savage Dungeon on turn 117 [marathon speed] spawned 3 Stoneskin Ogres... seriously, this -is- absurd.
As is is now, lairs seems to me like a "trap" for the player (and maybe even the AI; in the same game as above, I had no less than 5 enemy civs already defeated): either one lets them be (so they constantly spawn monsters) or one investigates (so that weak units get thrashed by spiders and giants, and strong units get poisoned, enervated, plaugued and the like). Spawned monsters are just too many, and given Civ4's combat (which I loathe), sooner or later you -will- get your first city conquered no matter how strong your defence is; on the other hand, producing units and sending them to eradicate lairs is usually a losing strategy, because they are well defended, are not always destroyed upon exploration, and even with scouts negative results are both too frequent and worse than the good ones (not to mention that some results, like the above ogres, are definitely game-breaking). Starting a game "near" (abstarct definition; see the centaurs) a dungeon, barrow or the like is usually an exercise in seeing your improvements pillages, cities razed and units built, promoted and then dying.
Suggestions:
Add some tech requirement for the spawngroups (quite aggravating seeing barbarian archers and axemen when you're still learning how to fish);
Categorize exploration result based on the exploring unit's strenght/level/tier (perhaps even a special promotion) OR turn, taking into account game speed;
Prevent units to explore lairs that are within another civ's cultural borders, or make those lair unable to spawn monsters, or make it an act of war (quite sound if you think about it -you can't pillage my improvements, why should you be able to destroy my dungeons?; also, exploring lairs "hoping" to get powerful monsters spawned that then proceed to destroy enemy civs CAN be a sneaky, cunning sort of realpolitik manouver, and should be recognized as such -could even be a feature of Council of Esus); as it is now, I bould graveyards andmy allies pillage my dead, which get quite upset and attack me for it O_o;
I conclude saying that spawngroups are a great idea, they add personality and fear to those pesky barbarians.