Although I tend to rely more on natural method myself these days. I don't go dogging on Mystery Method as I know it does work and where I got my start out.
You don't hear much about Mystery these days because, he had kids and left the scene. Specifically having a daughter, I'm sure her had some reservations about what he was teaching other men and might have changed his point of view. However I have heard as of recent he is a bit back on the scene and doing a bootcamp in Spain.
Aside from him not being in the scene to endorse his own method, it was also the problem of replication. As with multiple doing his method and general society becoming more aware of
PUA. You couldn't just run around using lines "Who lies more Men or Women?" as they would know you are running a script. However these days, since people don't do it anymore. You don't have the same risk of getting "caught" and can still get a high level engagement. However the point was to really understand the principals and structure of good openers and DHVs and create your own. However being who they are and laziness and convenience being what it was copy and pasting routines is often the default easy route.
Stilll think Mystery Method has it's place these days. Particularly with the popularity of the natural method there allot of guys lacking any technique or finesse out there. Particularly like on good protocol, like how to be a good wingman or how to be calibrated with the situation as when is the right time to sexually escalate . (Hence the accusations in media of
PUA being connected with Rape culture. Ie. Roosh and Julian Blanc).
As Pickup is an ART. As an art you need a mixture of technique and creative expression. Natural method is creative, but sometimes people can skip over the technical part. Whereas some people who did mystery method could be the other extreme of being too technical, acting like droned out robots stacking routines.
I mean if you learned to play Jazz and want to play improv music. It does help to learn the basics of music at the very least (Such as the scales). And then understand theory on how to harmonize with others. And from there having a basis how to free flow and express yourself well. However some people entering PUAs are jumping into jamming without even knowing the basics. (Particularly the basics of even Social Skills and protocol which is why they might be having trouble with women in the first place)