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You are flashing the chip directly so apart from inadvertent short circuits and such if it doesn't work you can just keep trying until it does.

As for wire length it all depends on how fast they clock the SPI bus when flashing. You'll probably be able to get away with 20cm or so without difficulty , I've driven SPI displays with that kind of wire length before.

What kind of errors?

Have you read successfully at least? By that I mean getting consistent dump.

A long wire is an antenna that will gather electromagnetic noise from the air and turn it into random signal on the line. Shorter wires will be less responsive and therefore less noisy, and you can also mitigate the problem by grounding everything properly. It's also possible that with the wires in that parallel ribbon, they may induce crosstalk on each other. If you want to be really careful, you could replace that big ribbon cable with an STP cable and ground the shielding jacket.

Also, a noisy/low quality power input to the Pi will produce noise in its circuits and ultimately the output. If you can, supply the power from something better than a wall wart.

The length of the wire is actually more an issue of capacitance and interferences. But these issues are easily mitigated by reducing the spi speed

Is the wire length problem some sort of latency issue or a resistance issue?

I think they were under the impression I want to install Libreboot on the Raspberry Pi. Maybe they didn't get a good look at the photo or how it was cropped confused them

Then why don't you?

Just don't do this to your only main device