Kinds of problems:
The power of cheap cards can be all over the place and in best case just empty your battery faster, in worst case damage the electronics of the CFast slot of the camera (heard that from the technicians of Angelbird which found that out when a client had troubles with their card reader).
Cheap cards might be everything but reliable.
Thanks, I'm sure you're right that they "might" be unreliable. We will see.
Why would you risk loosing your footage by using a suspiciously cheap card from an unknown supplier?
I guess I'd risk it, after doing a lot of tests and satisfying myself that it was a reliable card. The technology is surely not so different, and I have no doubt that the odd card might be subject to lower standards of quality control, but if it works, it works, right?
You can buy a SanDisk Extreme Pro 256 GB SDXC card ATM for a similar price which does reliably most of the formats (except Highspeed) the camera provides.
Yes, I already have the Sandisk 128GB SDXC 95MB/s which is fine for Q5 Braw 25p, but not reliable for slow motion, or usable for 60p in high bitrate situations, such as a recent shoot in the woods. Also, I much prefer to shoot 25p at Q0 if possible, and normally use Samsung T5 SSDs, but I'd love to be able to shoot without a tether.