For instance: you can't even slice "$@" which is the only array like construct specified by POSIX (excluding unsafe splitting), so you end up shifting and re-enqueuing with set -- "$@" "$1", which is unreadable.
Want to read a single character? Impossible with the POSIX read, but you can workaround with dd
etc.
All those workarounds to POSIX limitations are fascinating, but it forces a lot of arcane constructs, that's for sure.
And of course this is usually the point where you ask yourself if you've chosen the correct language for the task, but that's another debate.