Wow, this thread is becoming very concerning!
And thanks @WvdW for your reply, however, I think we are viewing this differently. You are being very specific about the technical details, but in my mind, if the remote file doesn't end up being the exact same thing as the local file, then "it didn't make it"! Whether that's the file never transferred, it partially transferred, or it fully transferred but it's corrupted in some way. You can break it down into as many technical pieces as you want, but at the end of the day, my file either made or it didn't.
To your point regarding using Insync tools to confirm transfer success (whether UI or command line), I don't really understand where that provides any peace of mind. If I am questioning whether Insync properly transferred all my files, then why would I trust the outcome of those checks?
For example, one of the transfers I did was around 1TB in size and during that transfer, I lost power at the house due a to a windstorm and the transfer was of course cutoff. When power returned, I restarted everything and Insync resumed transferring my files as expected after doing a full scan. However, like an hour or so later it showed that the sync was complete. So yeah, I call BS on that. There is NO way that it scanned, uploaded and checked 1TB worth of files in roughly 4 hours or so of total runtime. In addition to that, when I check the size of my local Google Drive folder against the size of my Google Drive storage and there is a difference of about 500GB. So yeah, I am super concerned now, especially after reading @marte 's response.
So maybe Google is doing some fancy stuff to detect duplicates or anything else along those lines that would cause a difference in file sizes and counts, but right now I am completely lacking any confidence that this is all happening as it should be.
Not the most scientific way to compare the two, but clearly, something is off.