Yes, the issue is the "supposedly".
We also got <15arcmin gearboxes from several chinese suppliers, we bought some samples to test.
We tested it on rated torque on test-bench and the backlash was >1 degree. And after some runs on rated torque it became 2-3 degrees.
We bought also some from Neugart (German company) and did the same tests and they were and remained in the defined tolerances.
So, our conclusion was that chinese don't really know how to make precise gearboxes.
Harmonic drives are good, I mean the original ones, but they also cost an arm and a leg, so they are costy unless you can get some second hand ones from a seller who do not know who much those really worth.
The size needed for our BF50 Z-axis a harmonic drive in that size costs about $4000.