While my horses are fine going out in the mud, I have decided after a particularly muddy, wet horrid summer, not to ride them in the mud.
Now I'm not talking about a couple of days worth of rain, or even a weeks worth. But that non stop , let's build an ark type of rain. So, with that said, the reason I won't ride in that deep mud any more is simple.
Most of my trails have ledge under them. When the ground gets that mud sucking texture, then the horses hooves sink deeper and sooner or later, that under layer of ledge, can actually slice them. This has happened twice now, once to each horse. So, I won't ride and take that risk any longer. Neither horse has ever "fallen" or lost footing due to the mud. That's never my concern.
If I feel I need to ride, then it's road riding. I have a nice dirt road just above the house that will due for a ride without the risk.
Be the kind of woman that when your feet hit the floor each morning the devil says, "Oh crap, she's up!".