If the merging process is stopped in the middle, the PST file that is created can be partially merged or damaged depending on where the process was stopped. This could cause emails to go missing, objects to be duplicated, or an Outlook data file that can't be used. To stay safe, you should select a solution that is both reliable and stable. I suggest using GainTools Merge PST Tool since it makes sure that PST merging is safe, while keeping the data safe the whole time.