I have been doing zazen meditation as 2-3 hours per day for 10+ years.
The way I see it, mindfulness practices and meditation gives long term effects because you are doing the effort yourself. It's not the mental state you are in during the meditation that brings the long term benefits. What works is your constant effort to look at your mind and notice what it does, even when you are distracted or feel vexed and especially then.
When you do 7 day zazen retreat where you meditate 10-16 hours per day, you feel pain in your legs, periods of intense drowsiness, agitation, strong emotions, boredom, etc. all this while sitting and looking calm outwards. Within all this internal storm you are just looking at what happens your own mind at the current moment. Attaching yourself to pleasant mental states you get in meditation is just waste of time.
The way these EEG devices work is by giving you feedback loop that gets you into relaxed state. That's not same as meditation. It can have good effects, but it's not the same.
One person drives with car 10 km every day and other person runs 10 km every day. The amount they travel correlates very closely, but only one person gets exercise.