It really kind of depends on the nature of the questions you're asking. considering they aren't seeing you face to face to really screen what's going on or any health concerns, they are relying on just what you tell them. It's kind of a toss-up I suppose. considering you have a health anxiety, it'd be hard for them to really diagnose symptoms without being able to look over yourself... but to have someone with a medical degree to tell you if you really have anything to worry about based on the symptoms you tell them, it could definitely relieve you of any anxieties.
I haven't really used any of those sites, just seen doctors in person, but if you choose to do it let me know how it works out.