Genjutsu will be like Taijutsu, something available to everyone regardless of chakra nature. Everyone will easily be able to master stuff like getting out of a genjutsu, but casting advanced genjutsu will require lots of training. What type of advanced genjutsu we'll have has yet to be decided.
Apart from that, advanced genjutsu will be a risky matter for the user itself, as using it as an offensive tool will mean that this user is specialized in genjutsu. Being specialized enough in genjutsu to be able to use it as an offensive tool means that either this genjutsu user has been playing for years and never died or lacks some serious skill in ninjutsu. If he has been playing for years, managed to stay alive all that time and train ninjutsu to an advanced level as well (this would be very difficult because of the genjutsu specialization), then you're in trouble. But I'm guessing in most cases this won't be the scenario and the genjutsu user will be specialized in genjutsu only combined with some very weak ninjutsu and taijutsu as well.
Sneak attacks would result in the genjutsu user's favour, catching you off guard. But if you're aware of him and ready to counter his genjutsu, he'll be at a big disadvantage. You could easily take him down with some good ninjutsu.
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