> Return value will be the sum of exit values of each command.
That's unforuntate design choice.
Exit status is 8-bit only, and some programs exit with high numbers¹, so the sum could overflow easily.
¹ For example, when a Perl program dies, exit status is 255.
The comment about motivation came from a bug report about parallel being "chatty": https://github.com/Homebrew/legacy-homebrew/issues/29060
Nothing homebrew can do about it, of course, but this illustrates one of the implications of parallel doing something unexpected; package managers have to field the complaints.