You are correct, there are a number of risks in doing a migration and you will need to assess what those risks are and what you are willing to accept for functionality. Just to give you background reference, I work for a company that has specialized in SourceOne and the predecessor EmailXtender since before it was owned by DellEMC, I have been working in email archiving for over 17 years, and have been working with archive migrations for over 8,
If this is a small amount of data (less than a few TB), the SourceOne Restore may well work fine. You will have to plan for a space consumption for the restored data going back into a native target if going back into Exchange. Be aware that what you have stored in SourceOne when you look at the archive volumes in the S1 Admin console is single instanced and compressed. If you have SourceOne Reporting turned on, you can get a better idea of what sizes could be coming out. When extracting from an S1 Archive to native targets (be that Exchange, or even to PST) size of extraction target will need to be at least double, if not 4x or 5x of what you see stored inside S1. You also need to know that a SourceOne Restore activity, while it will do the job, can take a good bit of time (i.e. a few GB per hour per mailbox, in this case per journal mailbox). If you have a single journal and say 20+TB of data, you will be processing for quite some time and will need to split the job up and bracket it by date most likely. You also will get a one shot approach, that while you can get a report of what does and does not fail to restore (with detail logging turned on) you will not get a “Retry” option. You also do not get a report of what a mailbox by mailbox breakdown might look like for a migration (again unless you have S1 reports turned on and drill down into the mailbox detail for the users). Customers that have taken the approach of using the Restore Historical activity have seen small amounts and small counts of users work, but if you have hundred or thousands of users with 10s/100s of millions/billions of messages, the restore activity will be overwhelming so be prepared for a lot of work and rework as you go through the restore.
Keep in mind, that while a migration solution (yes I have worked with TransVault for a number of our customers in addition to some other migration tools) can have a cost to do so, the cost is about the value of the data, not the cost of the box. So you do need to look at the risk assessment of what the migration must achieve, and what the costs are of your time to do the work, as well as the costs or redoing work, proving the completeness of the migration, and probing what users got what data, and what did or did not succeed.
There are also other factors such as chain of custody, that dependent on the market segment your company serves, may well cost more than simply trying to go it alone. If you company is subject to regulatory oversight, or has a pattern of being highly litigated, the real costs may not even be the actual migration, but the risk of losing data in the migration or risks from being fined or sued for losing critical data. Part of your risk assessment can and should include the consideration of what would it take to simply maintain the S1 archive in place until the data in it has expired, this does mean operating a dual system for a period of time.
A migration solution will give you the option to do a pre-inventory of all data, including active and ex-employee, and archives and journals. You can then decide who, and what (based on filters such as date) is to be migrated into the new archive (if that is where you are going) or back into Exchange or PST. You can also direct ex-employee data to either a different user account, a corporate oversight account, or even elect to go external (such as to PST or MSGs) totally off-line if desired. So have multiple potential destinations for the migration can be quite helpful if that is needed. Depending on the new archive destination, you may well be able to do a direct migration from SourceOne into the new archive (such as if you are going to Office 365 and want to put data into the Archive of a user mailbox).
Leave a note if you wish to discuss further in detail with someone that has been where you are and helped other customers make this kind of change.