Have been downloading from sites that link to "Upcloud" "Vidcloud "Mixdrop" "Voe" etc etc etc for some time, places like
If Voe has the video, it's the fastest and most reliable at this time, things change, upcloud or vidcloud can be a lost cause at certain times, then they come good, and not together, it's random.
And on certain series or movies ... or certain episodes of certain series you will get the same result .... mostly, as the original files uploaded to these servers and streamed the the sites I mention above have the faults in the files, if you download via a chosen server, sometimes an episode will be wrongly labeled and an episode missing altogether ... whether you download via whichever site/server, the file is the same when the same server is chosen and mostly all the server options have the same files no matter which site you use to access those servers and will if it's faulty it's always a waste of bandwidth ... regardless ... unless a different file without problems is uploaded to different server(s), also there's times when a particular server will for whatever reason simply be flaky and the download will fail mid download ALWAYS.
I've downloaded eps of shows via a torrent to complete a series, then found those files to be faulty also, upon downloading the rest of the series, I've found he corresponding eps that were good via VDH download to be good via torrent download and the bad also bad, so perhaps the files were (in those cases) sourced from a torrent download with the faults.
There seems to be different reasons why downloading certain video is problematic from these servers regardless of the site you access them through, sometimes it's a case of searching till you find a site with different sourced files to complete a season or get a certain movie etc.
I have a 2 season show that has eps from 2 different streaming platforms downloaded via VDH different resolutions and the .srt files I downloaded don't sync with the odd one sourced eps, some were recorded from a broadcast, have the watermark for the broadcaster, previews and splashes for other shows etc proving they were sourced differently ... sometimes ... no matter how many attempts to down load an ep or several eps would have the same exact problems, I've had mkv files that crashed my sound drivers when playing via vlc to a bluetooth speaker that after trying a bunch of fixes the only fix I found was to reboot.
attempting to fix some files via VideoRedo also fails, proving the source file was faulty as the same outcome when trying to fix even with second or 3rd attempts to download and fix the stream.
Problem with viewing Firefly via a file is all the software that converts to mp4 etc removes the deliberate resolution drops that are displayed on the DVD to give a sense of emotion etc, it's worth owning the DVD for that alone!
Good luck, you either learn to live with and find the determination to overcome these issues, or be frustrated and give up.