GNU cksum shows an error and returns an exit code of 1 if a directory is specified:

$ cksum dir file
cksum: dir: Is a directory
4294967295 0 file
$ echo $?
1

uutils cksum, on the other hand, doesn't show an error and returns an exit code of 0:

$ cargo run cksum dir file
4294967295 0 dir
4294967295 0 file
$ echo $?
0

The same applies to -a/--algorithm with all algorithms.