With the upcoming release of the Crius update, industrial POSes in EVE will be receiving a few sorely needed changes. Among these changes is the transition from a slot system to an activity system: this means that in Crius, rather than needing to install multiple modules to increase the number of slots available for industrial jobs, only one module is necessary, with job cost increasing as more jobs are created.
This leads to the question: "Why install multiple modules if they no longer provide more slots? Why not install just one?" CCP initially answered this question by stating that in Crius, multiple modules of the same type would provide a bonus to jobs running off that module, meaning a POS with several of a type of module would still maintain some sort of advantage over a POS with just one such module.
The "technical challenges" CCP Greyscale mentions stem from the train wreck that is the code for POSes in EVE. Many an update has gone by where the community has cried out for changes to the POS system, and CCP has regularly shot down these requests, citing "technical challenges."
CCP's reluctance to add new features to the already existing POS code is understandable: messy code means features become more difficult to implement, bugs are more likely to be present/significant, and all the features will need to be redone once new POS code is written.
But CCP refuses to tackle the bigger problem: many features are shelved because of "technical challenges" arising from this messy code, but the messy code is never redone. The only way to move forward from here, a course of action that has been a long time coming and is, in my opinion, many years overdue, is to completely rewrite the POS code and overhaul the system.
I don't believe we will see any major improvements to the POS system until this course of action is taken, and while the features included in Crius are certainly desirable for industrialists, the effects that these changes will cause remain obvious: it is pointless to install more than one module to run jobs, so industrial towers will be switched to small towers running one single industrial module and possibly several weapon modules, cutting fuel costs and making the game much more boring.