For what it’s worth, my personal experience doesn’t mirror this.
I used to own SR20 G1, which had the S-Tec 55X. After resolving an issue that many early 20’s had regarding “stick wobble” (which proved to be an easy fix once it was figured out), my 55X setup worked perfectly, for 1700 hours of flying. (The originally installed 55 had a capacitor blow which simulated a fire, and after I sold the plane the TC went out.)
My G3 has the Avidyne DFC 90. I like that setup a lot.
The DFC 90 has some distinct advantages. Speed envelope control is considered by many to be a meaningful safety advantage. Many people find the DFC 90 to be much more accurate in tracking the localizer, especially in strong winds, although in my experience both worked great. The DFC 90 has a “straight and level” button and an “AP” button that has the plane continue doing whatever it’s doing, and I really like both of those.
At the same time, upon losing the HSI (actually AHRS), the S-Tec 55 will continue flying in GPSS mode, whereas the DFC 90 won’t. Different people have different opinions on the safety implications of this. To me it’s a big deal. I’m not one of the pro-type pilots, and I’m more worried about spatial disorientation if things go crappy than I am about letting the autopilot bring the plane to a stall. Yes, I routinely hand-fly the plane in IMC and under the hood, but disorientation seems to be a common issue under high-stress situations.
Finally, in my observation, both manufacturers have instances of bad and good customer service in their history. S-Tec has awful expense and time involved in repairs, and seems to have exploited the situation when the backlight failed. Many Cirrus pilots are justifiably upset over that. My observation and personal experience, is that Avidyne sometimes delivers exemplary service (I’ve experience that recently) and also poor service (I’ve experience that recently as well and to this day am upset about it). To say that one manufacturer has a consistent record in this area is contrary to my observation and I don’t think it’s supported by the facts, zealous advocates notwithstanding.
Net, I wouldn’t disqualify the chance to get your setup working properly. At the same time, if there’s meaningful expense to getting it right a DFC 90 swap is a very good option. Finally, Autopilot Central in Tulsa are incredible. Ralph Stahley is the autopilot whisperer.
Whatever you choose, I’d be sure to get the thing operational. Whether it’s the S-Tec or Avidyne unit, a functional autopilot is a huge safety advantage.