I would like to inquire about the SharePoint Migration Tool (SPMT) that the Dev used.
We already had talks with the business people and said, "Yeah it still an active site, let's migrate it so we do enhancement afterwards". And so, here we are.
We are migrating 5 SharePoint (2010 that's what they told me) on-premise sites & their subsites to the online platform of Sharepoint. They want to migrate:
 "Numerous" Lists & its items << they somewhat want to repurpose for something
 Folders and files (mixed of MS documents, PDFs, zip files, etc.), and;
 [a] View permissions for the folders [b].
2 (small) sites have like 1000 files & folders. 3 of them have like 25,000+ files & folders. We rarely get this kind of task because just this year on-premise site environments will somewhat "put to sleep" as per Microsoft.
So the questions here are...
 How will the QA do their validations & explain the "OK report" to the business people? Because this is quite laborious for the QA team to do check each file & folder between the source & the destination.
What else can we use as the basis to evaluate them, so "this & that" are migrated successfully & not corrupted?
 Reliability of the tool? In terms of transferring data, files & folder. And what's not being transferred.
@TheMing_PH I have completed many migrations using the SharePoint Migration Tool. It has been very successful in almost all cases.
It is important to ensure the network connection between the source data and the PC running the migration tool is reliable. I have seen issues when using WiFi for example, so I run the Migration Tool on the Server itself if possible (depends on the Windows version).
When the SharePoint Migration Tool completes a migration there is a link to the migration reports. This shows what was copied and details of any errors.
You can also do a quick check of the item counts between the source and destination by checking the item count by looking in Site Content.