After 30k miles using clipless on the road, it's still the unexpected abrupt stops (typically at crossings) that bring me to within milliseconds from not getting unclipped in time.

MTB clipless on steep technical uphill is where the real fun is at! The challenge is in dropping to 2mph, maintaining your balance, picking a doable line, steering quickly, pedaling at a fast 90-100 cadence to maintain momentum, and fighting the urge to unclip at all costs, even as you feel yourself losing balance. Nothing brings a wider smile to my face.