Yes they are the next "big thing"... they've always been around just need to be utilized more.
Yes the muscles are trained differently. Both are amazing. A lot of complaints regarding those movements are regarding flexibility.
For instance in the front squat I find that most people are flexible enough to perform the movement easily. If you can touch your ear... you can rack the bar... just move your elbows in & bam you have the position. Most people that complain about their wrists hurting usually have weak abs and bend forward when they squat downward thus causing the bar to roll into their hands rather than be supported by their shoulders/upper chest/ clavicle.
OH Squats is...everything. Everything has to work. A LOT of flaws can be overcome through overhead squatting.
But you can't really add weight to these unless you have a solid back squat.
The back squat cannot be replaced.
I'll say it again just to be clear...
THE BACK SQUAT CANNOT BE REPLACED.