I have a hard time understanding how TM can backup hourly while a user is changing or moving files, particularly large files and databases
Time Machine essentially makes a list of the current files and uses that list to backup. So any new files created will be done next time around. Moving files seems to have no effect, for example keep a pdf open and move it to a new folder, it still stays open and you can make and save edits. On Windows you get a file in use warning. So moving a file during a backup shouldn’t matter.
I imagine editing files is the real problem, or more precisely if an app uses and edits multiple files and Time Machine doesn't backup the exact versions, just the versions it sees. And certain apps might not save edits to the disk but keep them in memory until you close the app, therefore causing even more problems. And so if you need multiple files of the correct version per app, but Time Machine only got the files at certain times then you might have a problem.
Since Time Machine insists on making full copies of files no matter the size (versus a more efficient copying of just the changes) editing a big file can make the file be even more out of sync. Now Time Machine does make backups to the local drive, so I image that this helps with copying an edited file, especially if there is some trickery where it delays writing to the file to help with backups and then frees the space later. But I also imagine that most people who don't have Time Machine problems don't talk about it so the result is that you hear about the problems only.
Time Machine doesn't choose the first backup of the day, the most recent is always chosen (otherwise what is the point of backing up every 60 minutes). The backups are trimmed in size once your backup capacity runs low, but they cull the oldest files first and I believe they also simplify backups where they only keep a single backup for every day older than a few days. In my case it keeps the last 20 hourly backups and then defaults to single days.
But since old files are really only for occasional reference they don't matter for the usual sort or recovery. It is also entirely possible that Time Machine was never designed for backing up extremely large files over a slow connection (doesn't make use of file deltas), so perhaps in your example they need to either look into a faster backup destination (SSD over USB 3 can be quite fast, on the order of hundreds of megabytes per second and the newest SSDs can reach gigabytes per second) or look into backup software for more demanding users, or both.
On Windows you have Volume Shadow Copy which facilitates making backups of files in use, so I imagine that Time Machine uses something similar. I have only had full on backup refusal problems but never incomplete restoration issues. Outlook may also be a special case, as the complaints about Outlook and large mailboxes are common. And a quick Google shows that Outlook likes to corrupt as you approach its mailbox file size limit. So this problem can be either in Outlook or Time Machine, but it can also be the extra fun kind where it is both and therefore hard to fix and impossible to find a solution online.
