Shuttle RTLS abort contradiction

In the Shuttle RTLS abort, the stack continues its ascent till SRB
shutdown, then turns around till its SSMEs are facing forward and fires
them to reduce its velocity, jettisons the ET and glides back to the KSC.
Fine and dandy till you consider what would lead to it abort its ascent.
If it's a problem with the SRBs, that would probably destroy the launch
stack.
So the most likely problems after that are either a problem with the
SSMEs or the guidance system.
But you are counting on both the SSMEs and guidance system to position
you correctly and slow you down so you can return to KSC.
So how exactly is this supposed to work?

Pat