I am involved in online/phone/email therapy for PTSD. I've been working with my therapist for one year now. I have found the experience incredibly powerful and successful, though not without unique difficulties. I think it requires strong written communication skills. My own therapist was in private practice, in person, for seventeen years before beginning online therapy. She maintains licenses in her state. She has promised if the service we use folded or had difficulties, she would continue working with me in another format. She has special training, and ongoing education, with an emphasis on PTSD.

P.S. As far as not being sure which services to take seriously: I would focus most on finding an excellent therapist- base your decision on a good person, not necessarily the company. Make sure that person is experienced and verified- you can check licenses, look at LinkedIn pages, google, etc., use a few methods.

Here are my thoughts, off the top of my head

PROS

Availability
Online professionals typically have greater availability - good for folks who want support in unusual hours like evenings, weekends, more sessions per week than one.

Session time
Sessions are not limited to 50 minutes, the format supports talking as long as one needs or wants to, from a few minutes to hours.

Credentials
Some sites, like LivePerson, do independently verify credentials and you can ask providers for their License numbers as well to confirm them with their state licensing board

Customer feedback
Experienced online therapists at some services have hundreds of reviews from many users.

Convenience
No commute, no dressing up, no leaving a session in tears feeling vulnerable. I have most sessions in the comfort of my own home, but can have them anywhere that has wifi or a phone.

Specializations
Easier access for folks in small towns/remote areas to specialists

Progress tracking/review capability
I have transcripts of all my chat and email sessions. I can review them whenever I want.

CONS

Cost
Online therapy is less than in-person therapy in some cases, but not usually covered by any type of insurance in the U.S. anyway

No Physical cues or reassurance
No hugs
No touch
No eye contact (unless you go with one of the services/individuals that allow Skype or other video conferencing)
I have to rely on *telling* my therapist when I'm crying, how I'm feeling, being conscious of my body language and sharing that

Reliability
Most online providers are in private practice and may leave without a trace if they don't have integrity, and that would be the end of the relationship. (Working with an experienced, established provider minimizes this risk.) However, this has happened to lots of clients that see therapists in offices too, in terms of sudden terminations, personal issues arising, changing practices and being banned from taking clients, etc. No guarantees either way.

Miscommunications
Email sessions, which I do often, leave more room for miscommunication since there is no body language, verbal cues, or real time interaction. This gets much better with time in my experience, as a trusting relationship is established.