As a former circuit board (and flex circuit) designer, I would expect vias in flex circuits to be about as reliable as in more traditional (rigid) laminates. Factors that would influence reliability include:
- The amount of vibration and mechanical stress applied to the flex over its lifetime
- The thickness of the wall plating in the plated through holes (aka vias)
- The quality of manufacture (a flex circuit built by a fab that has experience with the technology will more likely build a reliable part)
- The layout of the via pads themselves
In the case of item #4, I would advocate for a “teardrop” shaped pad, to smooth the transition from the trace to the via pad.
This will tend to reduce fracturing where the trace enters the via pad on the flex circuit.
Read More: What is Flexible PCB