In fact it's not a "known risk". It's an active area of research, with at least some findings contradicting your assumptions, e.g.:
> Our findings suggest that the traditional hold-up view whereby unions discourage innovation does not necessarily survive. When the voice effect is neither too strong nor too low, the unionized sector outperforms the market in terms of process innovation, while the effect on product innovation is strictly increasing in the voice power.
> Our findings suggest that the traditional hold-up view whereby unions discourage innovation does not necessarily survive. When the voice effect is neither too strong nor too low, the unionized sector outperforms the market in terms of process innovation, while the effect on product innovation is strictly increasing in the voice power.
https://docs.iza.org/dp14102.pdf