Lag can be a serious issue if you’re looking for visual cues when connecting your combos or doing feints. But if you can land the combos and practice your timing enough, most of the time you’re still in good shape.
The only two things I can’t consistently do online are combo into ultra and do her c.HP xx HP TK feint, c.HP xx MP TK combo. The latter I’ve connected against people online twice. The only time I connect with ultra is if I do it randomly. My timing for connecting an ultra off of a corner combo or a ground Burning Kick has always been slightly off due to lag, and the time in which you need to throw out the ultra is precise. Never mind that it’s horribly, horribly unsafe if you miss.
It’s kind of a shame, but sometimes there are things playing as Viper that you just might not be able to do online. I’m trying to overcome the threshold, but it’s difficult. Viper does require a lot more precision than somebody like Ryu or Sagat.