When using current uc_signup with signup, any signup that is set up for e-commerce ends up having the list tab (the list of signed up users) suppressed. It cannot be accessed by anybody, even user 1.
The reason is that uc_signup implements hook_signup_suppress to hide the regular signup UI, to prevent users from bypassing Ubercart for products that require payment.
hook_signup_suppress is called in _signup_needs_output(), and this is called by _signup_menu_access when determining whether to generate/allow access to the list tab.
I believe this is incorrect -- the list is an admin feature, not an end user feature, and does not need to be suppressed in this case. There is already a permission for it.
Is there another reason that list-tab is checking _signup_needs_output()? If not, I suggest simply removing that check from _signup_menu_access (line 489 in signup 6.x-1.0).