Dense ores will always make a mystcraft world unstable.
unstable worlds give random maluses like hunger, weakness, nausea, fatigue and slowness.
unstable worlds may also have random explosions (meteorite strikes), tons of lightning (none that will hit a lightning rod) and most famously, corruption!
Corruption with spread from block to block, turning those blocks into corruption blocks. Every once in a while corruption will remove itself from the world and drag every block above it down by 1 block. Great big craters form on corrupted worlds, bedrock isn't safe from this either. If you don't mine your dense ores fast enough your mine will literally fall apart from under you, leaving you floating in the infinite void. Note that corruption doesn't spread through air. I don't know if corruption only comes from originally spawned blocks or if bits of it can randomly appear, but if it's the former then floating castles in empty voids are a thing.