Why Did My Google Takeout Download Fail?
Usually a large single file over a browser connection. Smaller parts are far more reliable.
Big downloads over a browser fail more often than people expect. A dropped connection partway through a 50 GB file leaves you with nothing useful, and browsers are not good at resuming them.
Ask Takeout for 2 GB or 10 GB parts instead of one huge file. Each part downloads separately, and a failure costs you one part rather than the lot.
Takeout JSON Metadata Fixer takes all the parts together and reassembles the library itself, so splitting costs you nothing later. It runs on your own computer and is free for your first 500 photos.