Reset is a pretty wide term. First of all, the difference: Stable, Unstable and Nightly are three different things. Nightly universe corruptions are pretty common lately, due to dungeon names being changed and stuff. Unstable and Stable weren't touched yet, so the first time you'll see a corruption (forced reset) coming there will be when the new stable update is released. Alternatively they could push nightly to unstable first, which is unlikely, but in that case only the unstable universe would get corrupted. Then again, universes don't just reset themselves, you have to delete them manually. But in short, yes, in long term the universe will be reset. Player characters however might only become incompatible, which is pretty much fixable.
At the moment none, but in terms of versioning, let's take a recent example: Stable -- Obvious, core game. Unstable -- Content for the next update that's been verified through Nightly but might still be buggy. Nightly -- The test platform that's used before commiting to unstable, usually VERY buggy. But due to the fact that they're working on two - three updates at once in nightly right now, unstable is unused and is about the same as stable.
