A couple of years ago I did quite a lot of backstroke with a pull buoy and found I was much faster than without it. At the time I couldn't swim front crawl with a pull buoy at all. I just wallowed about in the water. Since then I found out how to swim with a pull buoy and found I was also faster on my belly. Recently I have been trying to find the same feeling without a pull buoy and have come close to my best times with the buoy, but have not equalled them. I know that the TI view is that they are not helpful in the long run but it seems to me that they can be useful as a diagnostic tool.
I have been doing some one-arm backstroke recently and find it useful if frustrating. I am sure that the problem with both front and back crawl, in my case at any rate, is longitudinal balance. Fixing it is another question.