For me a missed session is missed and can’t be made up for. I haven’t used EMS but I believe in one of Charlie’s books or DVDs, maybe the Vancouver seminar he talks about when and where it fits in (I think GPP). It isn’t a replacement for weights by any means. It’s a passive way to work the muscles (bypasses the CNS as far as I know).
There is nothing wrong with missing a weights session here and there. What you don’t want is to miss too many and then have to re-adapt to the stimulus. That is the beauty of Vertical Integration: it removes a lot of that phase to phase adaptation “down time” that is characteristic of classical periodization.