I've learned this whole modern forehand "thing" and letting my arm whip around in the last year. It's been an adventure.
I'll say that for me it's taken a fair number of hours of practice to get the feel of it. At first, when I did it correctly, it felt great, but other times I'd miss time it. I wasn't feeling the arm whip and I wasn't getting any power. Some days I'd really have it, and then I'd struggle the next time out. I kept at it, practiced on the wall, and now it's pretty automatic.
Relative to my old school forehand the two biggest improvements are greater spin and consistently hitting with more power - and they're linked. I can get a lot more topspin than I used to be able to. This is critical for getting shots to land in that would otherwise go out. I can also hit with greater power in all positions on the court. When I have to I can just stay in a totally open stance, reach across with my left arm and take the racquet back, and rip a pretty decent forehand. But without the topspin all that extra power would launch balls long.
As I've gotten better at hitting this forehand it really cool how much racquet head speed I can get without really swinging that hard. The racquet head just naturally whips around. I can control the swing path so that I can control the amount of topspin verses hitting through the ball.