"Nevertheless, Tether Limited states that owners of tethers have no contractual right, other legal claims, or guarantee that tethers will be redeemed or exchanged for dollars. On 30 April 2019 Tether Limited's lawyer claimed that each tether was backed by only $0.74 in cash and cash equivalents."
It seems that it is unlikely that Tether poses any risks, on the basis that anyone who cares can find out the whole thing is shady with even the flimsiest investigation. A risk that large and obvious can't in itself be systemic, because the system will insulate itself from the risk. Everyone knows.
The systemic part is when people start being able to buy Tethers without realising it. Eg, if the banks start selling them to retirement funds or something.