i also have only used Hitec servos over the years.. Over the last few years I have had a few 645MG servos fail, thankfully with no loss of aircraft as I found the fault while doing pre flight checks. One of my flying mates has been using the HD servos for the last few years and has had no problems with them, which had prompted me to use these in a large SE5a that I have been flying for about 6 months now and to date they are really good.. I would have no problems recommending them as a great alternative to the well known brands that most have been using over the years, and at a considable cost savings also....
Which part number HD servos are you using ? Just wondering if they are the same # that I have.