No evidence safe or
just little to no evidence of any benefit.
The key role of ubiquinone is within mitochondria. While taking additional can increase concentrations in blood plasma, there is no evidence of this changing concentrations within the mitochondria & clinical trials have not shown benefit. See: https://en.wikipedia.org/wiki/Coenzyme_Q10 & https://en.wikipedia.org/wiki/NADH_dehydrogenase_(ubiquinone)