I see a some juniors trying to do a modern ATP forehand and struggle a lot. Yet to see girls hit one, then again you won't see many modern forehand in the WTA, and if top level professional female players aren't using it then I struggle to see why junior girls would benefit at their development stage.
I don't see a sound modern forehand technique until futures level. If your intending to make tennis your profession and break into ATP top 100, it's probably valuable but not essential. Definitely essential in today's game if your contemplating Top 10. Gilles Simon got to No.6 without a true modern forehand, very 90's.
What I don't like about it for non-professional players is it has a lot of compound motions that require perfect timing in order to get high racquet head speed at impact. It engages a lot of wrist motion which I have yet to see many club players control due to lack of strength of flexibility. The angle of contract happens over a significantly shorter time frame, hence high racquet head speed, but this means the timing required to ensure the correct angle at impact is significantly increased. In effect it's a much riskier shot and requires much greater timing, flexibility of arm and strength. There are obviously benefits else it would not be used, but to me these benefits are not evident in non-professional game and obviously by lack use in WTA including the very top not a necessity in WTA.
Agassi does not have a modern forehand, he has a Bolletteri or late 80's forehand. As doe most of tho Top player through 90's and early 00. The Hewitt / Agassi / Courier forehand is much simpler in mechanics and as such I believe is far better for most non-professionals. All my ex-professional playing coaches have pushed the point that increasing complexity and timing requirements should not be done unless you need to (IE: mastered simplied shot and require it to add more pace/spin/work). It's like serving at 100% or red-lining any shot all the time, it loses matches. There are numerous professional coaches I've read or heard who recommend simplification over complexity at least until mastery is reached. Of course if your as naturally gifted a Federer and happy to train like him, go for it. But if your a club player then your likely holding your game back with complexity and higher physical requirements than required.