By definition, the nightly build is always unstable. By "stable", we mean no new feature are added, only essential bug fixes, and the code has gone through an extensive beta test phase. That is why the 4.0.x series is called "stable", and the nightly series which frequently new (and untested) features are unstable.
The hope is that a stable build is much less likely to have bugs, but "stable" should not be used to refer to the likelihood of finding few bugs. Programmers don't choose to put bugs in or not, the choice is whether to add new features or not.
Even if the symbol library table does not have bugs, there is no documentation about it, and few people have experience with it so getting help may be difficult.