The current release of MoStacks is reasonably stable, and chances are good that you can configure and use a database without encountering a bug, neither with the Windows nor with the Symbian program.
However, MoStacks is not "finished" in any sense. Most importantly some regions of functionality are not yet fully documented, e.g. the script language. Furthermore a number of functions are still missing. You find some details about this in the roadmap.
You could therefore call the current MoStacks release a beta version, but that would not quite fit the traditional use of the term "beta" for a program that is essentially feature-complete but still has bugs and rough edges.
That's why this release is called experimental.
If you decide to accept the invitation and "experiment" with MoStacks, you can expect a program that is probably stable and bug-free enough for productive use, if the stack structure is not too complicated, and the number of cards not too high.
However, if you push the program to its limits you will probably run into bugs. The myriad of possible stack configurations that are possible based on the considerable number of features leave a lot of room for bugs to hide. (This must be a general problem for programs that are not straight applications for a single purpose, but something like "platforms".)
If you do run into a bug, there is some probability that you will loose a stack by corruption, with a stack file that has a "corrupted", invalid structure and cannot be opened anymore. To protect against this danger somewhat, MoStacks on Symbian writes backup files. Read more about this in the chapter Backup Files.
The UIQ3 MoStacks Manager currently does not support keyboard-only operation: No support for P990i flip-closed mode, serious UI problems on the keyboard-only UIQ3 phones Z8 and Z10. Read more about this in chapter Keyboard-Only Operation.