It is far more harder and the community often sucks.
Also, multiplayer are generally harder anyway, since casual always get owned by PGM.
Then, these are often real-time, which could make uncomfortable those who prefer or are more used to turn-based.
At last, there is a wipe every 6-12 months that will delete your character and all your stuff.
Otherwise, there are many things to do, too much things to do.
Personnally, i stopped Fonline because i spent too much time on it, nearly missing my university exams.
So it is better for me to avoid it entirelly if i want a life.