The worst case with a tow line is when the cleat pulls out. That will come
back
like a missile and can punch a hole in your boat.
It can certainly spoil your day.

I figured that a cleat coming back on a nylon rope was a bad thing. Nylon
is good for towing, since it has some stretch and will even out the strain
of waves and uneven pulling. I haven't done much heavy towing, but I
thought that if I ever did I would tie a fender at the middle of the tow
line. if the line broke or a cleat pulled out the fender would act as a
brake to absorb some of the snapback of the whole line. Of course if the
break was at the fender both ends would snap back towards the vessels.
BTW, you're not supposed to tow with or from a cleat. Use the bow eye and
the stern rings. Rig up a yoke to even out the pull between the two stern
rings, which then share the load and help keep the towboat straight(assuming
that you don't use a slip knot).