Lage der Nation had a really good summary of Libra and summarized a lot of the concerns, especially on why this is not a case where you can let them do their thing, and regulate afterwards, when you've gathered some experience:

https://www.kuechenstud.io/lagedernation/2019/07/26/ldn150-p...

Of particular concern is the scale to which this can be bootstrapped by having the tech giants involved.