The example itself is not incorrect. It is just an example to show how the foreach works, not meant to be a full command on itself. Usually I don't have newline characters in files either, so that is not a concern for myself. If I would want to be sure, then yes I would use zero option. But its good to point that out.

You can also avoid cat since you aren't actually concatenating files (depending on file size this can be much faster):

while read -r url; do echo "download $url"; done < urls.txt

Usually this is the way. Once you enter xargs’ world, you lose access to your shell aliases, functions, and un-exported variables, which will often bite you in the ass.

You should use -r option for read command to preserve backslashes. I was using while loops before too, but wanted to have a compact single command replacement. And doing it with a while loop as an alias (or function) didn't work well, because the command has to be interpreted. xargs does exactly that, as it is designed for this kind of stuff. Other than having less stuff to type, I wonder if there are benefits from one over the other while vs xargs. In a script, I prefer writing full while loops instead.

Literally no indication of that