I don't mind having a filesystem that works reasonably well on most devices. But I would consider "being able to append to a file reasonably quickly" as part of

I don't mind having a filesystem that works reasonably well on most devices. But I would consider "being able to append to a file reasonably quickly" as part of that "reasonably well". Note that it's entirely possible to have a filesystem that's actually several different filesystems with a magic selector. (I mean, block sizes and reserved block percentages and inline inodes already are along those lines)