Bad actors will lift old exploitative models into Libra's ecosystem: payday loans with huge interest rates, wallet services that force a deposit and allow you to overspend and charge fees when you do, sign up people for accounts and services that they don't want and make commissions. Or maybe just wallet/business services that straight up scam people and disappear with no repercussions. The thing you set out to kill will just come back with a new flavor. Plus it's even easier now since you can do it all with a single click!
Then someone has to decide what to do with bad actors involved with Libra and allocate resources to actually enforce some actions against them (legislative, the cops, server bans, ISP bans, etc.). Now you've got those services paid by tax dollars dealing with a corporate currency and taking time away from other enforcement actions.
All of this is going to happen at the juicy, vulnerable threshold where the technical controls interface with the human element. I probably won't ever hack Libra/Move itself, but the human part is still there. If I can actually manage to get away with the crime and transact with a fake identity, it can't be reversed.
Bonus: that same $40 phone stops getting updated by a phone carrier and some drive-by malware swipes all of your LibraBucks with a corrupt text message and no one can reverse the transaction. Or maybe that Bloomberg supply-chain attack article will finally become a reality once there's enough economy behind Libra.
If this is widely adopted, it's going to have a pretty steep learning curve and plenty of real losses for everyone.