Playbin does not check if the linking to the app supplied sinks is successful.  See uses of gst_element_link_pads in gst-plugins-base/gst/playback/gstplaybin.c.
The problem with this is that it blindly assumes that the sink pad is named "sink".  This is probably fine for the converters it creates, but not for the user-supplied sinks.  I'm under the impression that element and pad names are mainly for easing debugging and documentation, they do not matter to the framework.

Solution: Check link success at least with an assertion or throw an error which is even better.  Link elements using gst_element_link, which does not depend on secret pad naming conventions if I'm correctly informed.  Should the current naming requirement be kept, mention it in the docs.

I just spent hours of wading through debug output to find out why an app would freeze over after a change so trivial I couldn't remember it or tested after including it (a simple pad name change!).  That's why this bug is such a bummer, you end up with a pipeline that never gets out of preroll without any error whatsoever.