There are also many Linux distributions that do not install by default all the POSIX utilities, but only the minimal set that is needed to bootstrap the system.

On all such systems, it is very easy for the user to install any missing POSIX utility, but it is also easy to install any non-POSIX GNU utility.

So not even xargs is certain to exist by default on all systems.

Moreover, POSIX xargs is restricted to execute sequentially all processes.

Any use of xargs for parallel execution is non-POSIX, so in that case there is no reason to not use "parallel" instead.

    parallel --embed > parallel.sh
 

Then store that in your source repo and use it wherever shells are used!