> Can also do `oflag=direct` and it'll just skip as much of the caching as it can.

Correct, and that's one more point for dd compared to head/tail (which are fine commands by themselves).

But wouldn't help much in my example, where I used an (very) old "high speed usb 2.0 stick" with 4 MByte/s write speed to demonstrate the difference between buffering and actual writing.

Thats when it matters, with cat it would cache the entire write. If you don't want it to cache everything and have no idea how much time is left or how fast it is writing, and then wait an hour for it to unmount instead then cat is fine.