If you read the ZF brochure BMW's problem becomes clear:
Ideal for vehicle manufacturers: The steering column and servo unit are delivered to the production line ready-to-install. (
http://www.zf-lenksysteme.com/upload...ctric_E_09.pdf)
It's most unlikely your garage is able to repair that highly integrated unit. Even BMW most likely can't and certainly will not do it.
The only feasible way is to replace the whole part, which means disassembling most of the front gear. Presupposing ZF mastered the "variance" in their production meanwhile. And that takes a mechanic who knows, what he's doing.
Nowadays modern cars are mostly patchwork of parts from different suppliers. The more integrated the supplied parts become, the less influence the car manufacturer has in case of belated technical faults.
Similar to the byer of the car, the manufacturer itself then has to (re-)submit a warranty claim and wait for a fix.
According to BMW my F30 is repaired on on his way back. The head of the dealer's garage told me, that along with the car he will get a detailed report, what parts have been replaced. He is going to show it to me.
I'm curious what they did and of course if they really fixed the problem without generating new ones.