Those games aren't free to play they are more P2W pay to win.
Yea sure they are often graphically more beautiful and gameplay vise more challenging but I hate the fact that if someone has more cash then me can easily buy some form of an advantage over me and beat the crap of me.
For that I can just go live my life.
So to conclude yes sure they are graphically stunning, gameplay innovative and story interesting.
But to call them free is really going to far. Those studios make more money then WoW and Blizzard.
I remember when there was a article which said that Zenga (maker of "free" facebook games) was amongst worlds top 3 most wealthy studios.
So I mostly play those kind of games for month or 3 and then I stop when I come to the lvl where I need to cash out to be equal to others of my lvl.