From "emacs -Q --eval '(eshell)':

   ~ $ (eq 'foo nil)
   Expecting completion of delimiter ' ...

   M-: (unload-feature 'em-extpipe)

   ~ $ (eq 'foo nil)
   ;; No output, since it returns nil. This is correct.

(As an ironic twist, you'd normally be able to run "(unload-feature 
'em-extpipe)" as a command in Eshell, but this bug prevents that from 
working.)

I don't fully understand all the details of the code in 
lisp/eshell/em-extpipe.el yet, but I think this is the culprit:

                          (while (or (eshell-parse-lisp-argument)
                                     (eshell-parse-backslash)
                                     (eshell-parse-double-quote)
                                     (eshell-parse-literal-quote)))

In particular, since there are 3 single-quotes, 
`eshell-parse-literal-quote' throws an `eshell-incomplete' error, which 
gums up the works.

The attached patch resolves the issue for me, but I'm not sure if it's 
the best strategy. If possible, I think it would be better for 
`eshell-parse-external-pipeline' to solely focus on finding the external 
pipe operators ("*|", "*<", and "*>")[1] and then for 
`eshell-rewrite-external-pipeline' to prepare the command string to pass 
to sh. This would also have the advantage[2] of making it possible to 
support a richer set of Eshell features with external pipes, such as the 
following:

   ~ $ echo $(message "[%s]" "hi") *| cat
   zsh:1: command not found: message

(If you remove the "*", this outputs "[hi]", and it should be 
technically possible to make this work with external pipes too, provided 
it executes the Lisp code before generating the command string for sh.)

[1] See `eshell-parse-delimiter' for an example of how that might be 
implemented.
[2] Well, maybe there's room for debate on what the right behavior here is.