It is a horrible language design, which was impressed upon me after implementing it for my shell Oil. Here are some pathological cases I pointed out:

${####}: http://www.oilshell.org/blog/2016/10/28.html

Each # means a different thing.

${foo//z//}: http://www.oilshell.org/blog/2016/10/29.html

There are three different meanings of / there.

"${a[@]}" -- http://www.oilshell.org/blog/2016/11/06.html

You need 8 punctuation chars in addition to the array name to correctly interpolate it. Every other way gives you word splitting issues.