This issue has nothing to do with a gps, firmware upgrades always carry the risk of failure, it can be almost anything, assuming causes is literally just inventing them, the range is so wide that there is simply no way you can know. Here's a small list:
connection drops for a millisecond, could be many causes, usb cable defective, poorly seated, etc, wifi connection, if used, fails or distorts, whatever.
firmware operating system bug, or a battery state vs firmware upgrade issue, so many things there it's almost impossible to ever know.
hardware issue, this can be a wide range of things, could be a bad cell in the flash memory that the firmware is stored in, and the new version occupies that space, and presto, brick.
Now, if Garmin explicitly tells you to upgrade firmware, they really should always replace bricked units, but that tends to get kind of expensive.
The general computer world policy on firmware upgrades is to do them if you have an issue that the new firmware resolves, and don't do them if you don't. The same goes for computer BIOS upgrades, which is also a form of firmware, that is also very prone to failure.
Every time I do a firmware upgrade I consider it a very high risk action, and am always prepared for it to fail. It would be nice if low end consumer trinkets like a gps could be working at military grades of reliability, but they don't, otherwise they'd cost as much as military grade electronics, which are tested for probably, I'm guessing, 100x better reliability in the chips and circuits.
When I tell a client that I am going to do something with their firmware, I always tell them that there is a significant statistical risk of failure, that may or may not be covered by the warranty, or that warranty may or may not be honored, it's highly variable.
My favorite case of upgrade failure was on an IBM / Lenovo laptop, I used the windows firmware upgrade tool, turned out it had a huge bug, that consistently made the bios upgrades fail, thus bricking your shiny new laptop. Known issue, not resolved, no support from company, that's how it goes. Turned out the only reliable way to do it was using a live cd or usb powered upgrade. And that's not a trinket, but its' still dead and the company offers no support or replacement when their firmware tool fails.
Just how things go in the computer world, it's a low margin low cost production process, no matter what their advertising might try to make you believe, and the less you pay for something, the less they spend on the hardware and software. Knowing the realities of low end hardware and software is why I would never even dream of relying on a gps in the backcountry.
For the technically inclined, 'firmware' refers to a package containing the device operating system, and the software and user tools that runs on that operating system. A firmware upgrade literally reflashes the memory that firmware lives on, which means there is this moment when the firmware has replaced itself, and all of the normal processes are not working, it's a very short moment, but it is there, and if anything goes wrong in that moment, or in the process leading up to that moment, bang, brick. All devices that use firmware have this reality, and this problem has always existed, it's just better understood in communities that do a lot of stuff with firmware, the basic notion is that what is controlling it is replacing itself, and at a certain point, any error at all can destroy the replacement process, and because the device is so simplistic at that level, there is no way for you to access it unless you have access to extremely specialized tools, and even then it tends to fail.