I would like to distro hop to EndeavourOS, and it has been suggested that I use rclone to copy my home folder to an external SSD. Unfortunately, I don’t have an external SSD at the moment, but I do have a phone that has plenty of free space. Is it a good idea to back up my computer files to an Android phone? My gut says it’s not a very good idea, but I’m not sure why.


Short answer: No.
Long answer: absu-f-ing NO. Whatever you put on a smart phone should be considered compromised.
Buy an external harddisk (Not! an SSD) for that.
Why not an SSD?
Not the original commenter, but possibly because SSDs are eventually volatile.
SSDs store data by trapping electrons in cells to represent your ones and zeroes, and when the disk is unpowered those electrons can eventually escape, causing data loss.
This may take years to happen though, so if you use the disk frequently you are unlikely to experience it.
That said, the time to unpowered data loss gets worse the more cumulative data you have written to the disk over its life. SSDs that are extremely badly worn out could lose data in months, not years.
Traditional spinning disks don’t have this issue as the magnetic storage doesn’t depend on power to stay magnetised, so it’s a better choice for archival storage.
Because SSDs (and other flash-based drives) are bad backup media. If not powered for longer time, they can lose data.
Op is distro hopping. I don’t think their data is going to disappear in the week they’re fucking around with their computer.
It’s still the safer option.
IIrc, because unpowered SSDs loose data on the long term. Yet, simply for backing up and restoring some days or weeks later, they should be fine.