IEEEeqnarray is a lifesaver during those cases where standard AMSmath/mathtools environments do not fit in.
This document explains all the commands, environments use-cases and variants thereof.
Describe the solution you'd like
Full-range support for all the commands/environments as in standard equation, align etc.
Describe alternatives you've considered
Stay away from IEEEeqnarray, since not many know about it. But this is an official IEEE tool, and sometimes is a cleaner workaround for long 2 column equations, spacing, alignment etc.