Can Simon give a success or failure based on essential text missing?

Yes! The Filters feature includes Find Required and Find Unwanted default filters.

So for example, if you do a telnet Script-based check, you'd want it to be considered a success only if this is output:

Escape character is '^]'

Then if that text isn't found (as would occur if telnet couldn't connect to the server), Simon will log a failure.