I agree with all of Charlie’s points.
From my personal experience, if I have the time and energy, I am doing massage whenever I can. I just find it more effective overall and I can tangibly feel the changes that I effect throughout the treatment.
However, there are many instances where I am using the Globus EMS or similar devices (particularly when I’m trying to help numerous athletes at once):
I have lots of people to work on and I need to prioritize who needs hands-on work and who can benefit from use of EMS.
Someone is so locked up (extremely high tone) that it would take me an extremely long session to bring the tone down. I can hook them up to the EMS to do some initial tone reduction while I’m working on someone else.
After a hands-on session (maybe 2-3 hours later) where we have loosened up the muscles and restored resting tone, we can use the EMS unit to work on re-educating the muscles in question. This is very common with the Vastus Medialis, where the entire quad was locked up and knee pain was present. When we loosened everything up, there were still problems firing the VMO, so EMS helped to jump start the situation.
If you have access to both (massage and EMS), by all means use both tools.
Additional to point two, which appears to contradict Waldemar but W was referring to EMS for Str development and not treatment or pre-treatment.