Online counselling can be invaluable if you cannot get to a therapist personally. It can help if you are too embarrassed to ask for help, or if your partner refuses to acknowledge that you need help. It is possible to get excellent help online by visiting as many sites as possible to get an over-all viewpoint. I would still advise talking to someone face to face as they can help you personally and ask relevant questions.
This can depend on the quality of the program, and the person using it. The downside of online programs is that they can’t be personalized to you or your situation, so try to find one that seems to address the specific needs you have or problems in your relationship. Sometimes, there are programs available for free that an individual or couple may be able to try for free as test subjects. One may have to do a bit of research work to find quality programs; be wary though, those advertised for free are usually either scams or else they may just waste your time. Reputable marriage therapists with their credentials listed (like Ph.D), are usually stronger sources; stay away from anything you can’t prove is legitimate.