I haven't used any online coaches but I am using Carmichaels book "The Ultimate Ride" as a guideline to get back in riding shape after my knee surgery. I like the field test that he uses, easy to do at home and the ranges he has set up for heartrate zones. They are much closer together than the ranges that most people use.
I had lost muscle endurance after my surgery and I would try to ride at the speeds that I was used to. Without the endurance I was merely wearing myself out. I did the field test to determine my heartrate and found that it was much lower than I was used to. So I carefully watched my heartrate when I rode and didn't get as tired. I also do many of the workouts spelled out in the book.
Something must be working as I'm now riding with a higher heartrate, longer miles and feel stronger than before.
I'm thinking about trying a coach for next year and probably will use Carmichael or someone trained by him.