Safety -- servo vs. stepper

I have a choice of using stepper drive or servo drive for my next
hexapod. My concern is how to cope with safety issues.

My previous hexapod is stepper driven, with fairly slow maximum
speed of just less than 1 inch per second. My next hexapod will be
faster, with a designed intention of fast movements of about 3-5
inches per second.

With my hexapod design, it is very difficult to have limit switches
on all axes. In software, it is easy to limit motions in various
ways, such as to prevent any joint angles or actuator lengths from
being exceeded. It is not easy to place a fixed limit switch on an
actuator to serve as an E-stop of last resort.

Under hardware fault conditions, it seems that steppers are safer.
In particular, most failures in the high-power drive system will not
result in actuator motion. With a servo system, it seems that an H-
bridge failure is likely to result in uncontrolled movement.

Has anyone experience with drive failures, and the reprocussions? I
was about to order servomotors, and my stomach went cold as I
imagined the danger of an sudden, RAPID, uncontrolled movement of a
50 pound mass moving in 3D.

Any thoughts or suggestions?

-- Carl