I'm not sure on specifics of why, I know less than most children about electronics so please take this with a grain of salt BUT I've attended a few hydronics training courses and diagnostic seminars where this subject was addressed. Not particularly with OWB, but the closed systems with cast iron boilers and radiators apparently wreak havoc with ECM circs due to the particles floating around in the system. Call me paranoid, but after seeing some of the pictures and hearing some of the chatter at these meetings I'm not going to be the one to recommend an ECM in an application where only $2 a month is saved with the chance of killing the circ. I've put a few Alphas on stainless OWB and really like the technology, but the mild steel units scare me unless there is another means of collection such as a mag filter.
Very good point on the pump sizing though, the 007 is almost never a good choice