Complexity depends on the implementation. For example, my current multi-way system is fully active and the system I am building has passive mains, but I will still be using a minidsp nanodigi 2x8 to manage all the speakers. This might be more complex than most would like.
However, Parham actually implements his flanking subs passively by simply choosing the right value inductor that gives him his desired low pass. He thinks of it as a "distributed" 0.5 way helper woofer, much like how other tower speakers often implement 0.5 way woofers. The difference is that it's not in the same location as the mains. If going passive, thought will need to be given to level matching. I don't have the knowledge/patience to do it passively, so I will be trying the dsp route.
All-in-all, no doubt, it's complex. Multi-sub is a bit complex already, and this doesn't help - in terms of complexity. I'd say that this is something only an enthusiast or hobbyist would consider, much like multi-subs. Definitely not a lifestyle solution.