Both scenarios were part of my training in South Africa. I have never
personally come across a situation where spoilers failed to open,
although I can certainly think of a number of scenarios where this
could happen. I do know of one where the spoiler handle on a Grob 103
broke after they had been deployed and jammed full open, resulting in
an off-field landing.

I was always taught to deploy spoilers close enough to the field that
you could make it if they jammed open and to make a longer circuit
with a moderate slip to landing if they jammed closed.

The Discus 2 slips very nicely - perhaps the double dihedral helps?

Mike