Takes more knowledge of betting patterns to learn your opponent as that is the only variable you can get from an opponent online.
Whether online or live, your long term profits will come from being able to recognize your opponents play style, their betting patterns, their tendencies in different situations and then exploiting them while at the same time watching your own playstyle, betting patterns and tendencies so as to not get exploited yourself.
An opponent's physical demeanor may help in some cases, but focusing on the above is what will carry you to success in the long run whether you play online or live or both.
This is true. Regardless of whether you can read their faces or not, you can discern which actions to take if you are able to recognize their playstyle. However, against an erratic opponent who conforms to no principle, it's difficult and you must hope for luck on your side.
Obviously though, the more you play against a person, the more you start to form patterns on what they do, but it's the same both ways (so you must be careful in how you play)