It doesn't support a lot of very basic things, like variables

    $ TEST=1
    $ echo $TEST
    $TEST

Interestingly, in the example they show:

    $ cat README | while read L; do echo "README: $L"; done
    

...and yet:

    $ read L
    /usr/bin/read: command not found
    $ while true; do break; done
    /usr/bin/while: command not found
    $ echo "quotes"
    "quotes"

Seems like a horribly backwards approach to implementing a posix shell...