I haven't worked with PM7 (yet), but my experience with PM6 and below is that it is the equivalent of
fdisking and re-creating your partitions (without destroying the data on the drive).
Repartitioning should not cause any instability that was not already on the system. If the OS or software
was having some problems with the file system before, then it might show up more after a repartition, but
that would not be due to anything PM did itself.
It would also not slow down the system, although some partitioning operations can change the cluster size
for a given drive, which can affect how the OS reads and transfers files.