I wonder how many of the reports of TM/Mojave problems are caused by TM wrongly reporting times, dates and sizes of backups?
My computer is a Mac mini with MacOS 10.14.2 and my backup drive is a Samsung 1 TB one, partitioned into two equal 500 GB partitions, one of which I use with Time Machine. It is Mac OS Extended (Journaled) formatted.
I last backed my HD up onto this backup drive relatively recently and my usage of the computer has been routine and modest (emails, a couple of small document downloads, etc.), so 'Preparing Backup…' should not have lasted long. Yesterday afternoon at about 15:35 I initiated a new TM backup, but, to discourage hold-ups, I deleted the 'in progress' file from my backup drive and allowed Spotlight to do a complete indexing first.
'Preparing Backup…' lasted about 5 hours. From about 22:30, TM began to show 'Backing up'. At 23:20 the amount to be backed up was shown as '3:80 GB of 6.93 GB' and remained unchanged thereafter; the time till completion was shown as about 40 minutes, but gradually increased over the hours. After about 5 hours, I stopped TM and disconnected the backup drive.
This morning I reconnected the drive and examined what had been backed up. I obviously could not examine every file, but as far as I could tell everything had been completely backed up and was up to date. There appeared to be no way of reliably ascertaining the time of completion, but it was probably before 23:20 last night, relatively soon after the start of 'Backing up'.
I conclude that in my system TM does eventually do a complete backup to a drive formatted as Mac OS Extended (Journaled), that, however, it takes its time to start backing up after starting 'Preparing Backup…', and that it shows misleading information while backing up and even when backing up may already have been completed. Whether 3.80 GB or 6.93 GB or the difference, 3:13 GB, were not backed up I have not been able to ascertain, unfortunately.
I have updated my report in the Bug Reporter accordingly. I really think that that is the best place to report these problems and failures, because the Bug Reporter allows one — indeed, asks one — to attach a 'sysdiagnose' file containing a wealth of information about the inner workings of TM, generated while TM is working, by means of a Terminal code or a combination of key-strokes. I know that the Apple engineers read these reports, because when I first submitted a bug report about TM but failed to generate and attach a sysdiagnose file, the engineers replied within the Bug Reporter, asking me to repair my omission.