Look into exec autodoc (exec.doc) and search for:
SAD/--Overview--
Basically this explains SAD is not a full debugger but interface for external debugger to access the amiga thru 9600 baud 8N1 serial line.
KS 1.x/2.x had RomWack that was real debugger, but afaik it took too much space (3.x was running out of 512K, some stuff was removed to gain space, for example RomWack and shell H-bit support. For more details, read amiga release notes, available on DevCD) and was replaced with SAD for 3.x.
I haven't seen a debugger designed to use SAD. Probably there were some, though.
