I don't know how current you are with EMS research. However, in 2010 there was a world conference on the subject with the foremost researchers in the field. One statement alone from the summary article* refutes what you wrote: "Chronic effects - Strength training by NMES does promote neural and muscular adaptations that are complementary to the well-known effects of voluntary resistance training".
After that, two more recent research articles (2011 and 2012) have pointed out what allowed certain research paper to obtain good results, and why others did not. See my previous post. You also state: "producing high enough forces with Estim to get strengthening is typically pretty painful". That is also not true; it depends on what parameters one uses and how good the machine is at producing those parameters.
Article downloadable from my website research page
Maffiuletti NA, Minetto MA, Farina D, Bottinelli R. Electrical stimulation for neuromuscular testing and training: state-of-the art and unresolved issues. Eur. J. Appl. Physiol. 2011;111(10):2391–2397.