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.
Just a couple tweaks to that.
Spontaneous Explosions and Meteorites are actually separate. This also means you can have both.
Spontaneous explosions are roughly creeper sized explosions that occur randomly. Meteorite strikes are massive meteors that plow into the ground exhuming a massive hole ( something like 25-50 blocks in diameter )...and leaves behind a bunch of ores as well.
What you described as "Corruption" is "Black Decay". That's the only one of the decays that behaves that way. The others will not remove everything. I don't remember all of the colors, but I do know white deals damage to anything walking on it.
You are correct that a floating castle is safe from Black Decay ( Unless this was changed). This is actually why explosions, meteorites, and sun burning were added.
Other Decays however can definitely appear in air, when next to existing decay. So, they can't appear in the middle of the sky, but they can grow out of the already corrupted ground, eventually making it to your castle.
It should also be noted that you can destroy decay fairly easily, so if it's slow enough you can usually keep it at bay for quite a long time.
