I wonder how well would it perform when used in place of laptop's system disk (i.e. with tons of small files and very random read/write access patterns).
What's the seek time ? I use tiny usb keys as live image source, even as stateful system half the time, and it's a bit touchy. When the cache gets full, the kernel will get stuck in very long syncing mode, so much systemd starts to complain. On non recent (say 2013) usb keys, it will even trigger kernel error messages and lockups.
Unfortunately, this has been my experience with all SD cards I've tried. Even ones sold as high-performance are just annoyingly slow to use as a root FS.
They also seem to be extremely unreliable when used with even a modest write load, often failing within a year.
As a root FS, agreed. As an expansion, I have had really good luck with the 256GB microSD card in a low-profile SD adapter on my MBP. When I'm doing screencasts or whatever, I happily dump my recordings out to it and, as silly as it is, it's one of my favorite pieces of tech right now.