Hearing so many stories, I am absolutely staggered at what the % is likely to be.
In Europe, probably well over 50%, due to the dire state of the avionics business here. In rentals, probably nearer to 90% 
My KFC200 is booked in to get it back up and running, so not chronic just waiting for my slot at a shop nearby with the correct diagnostic tools.
Varies, every DA40 and DA42 I have ever come across or flown have all been fully working. In contrast, much less on Arrows and Duchess and the like. It is more a question of equipment age I think.
Mine was nonfunctional when I bought it, and sounds as if it had been for many years. The previous owner warned me that “the autopilot will kill you” because it would bank hard to the right when engaged. Went by Straubing a few days after I bought it and they fixed it good as new in a day and a half for a few hundred euros. Cheapest trip to the shop so far!
The biggest problem with getting broken autopilots working is probably the need for expensive test sets that are specific to the given manufacturer (if not the given model).
Ultranomad wrote:
The biggest problem with getting broken autopilots working is probably the need for expensive test sets that are specific to the given manufacturer (if not the given model).
My experience from a decade ago seriously suggests the biggest problem is finding an engineer with the knowledge and skills to fix the issues. There is really no substitute for experience but it seems experience is no longer valued and just keeping swapping out parts until the problem goes away. Especially true for older systems.
I found feedback from pilots with similar kits and glitches to be very valuable
The likes KAP & KFC suffer from gremlins when the kit is old and aircraft is not flying often (maybe explains why school DA40 with KAP that works while KAP in private Arrow is usually out of service), also lot of maintenance outfits do great job on propellers, engines & airframes but for avionics one has to really stay away unless they have deep pockets for experimentation
To answer your question, indeed it must be close to 90% for the older types for hire, prob a bit less for private owners. But for Diamonds, Cirri and the more recent Cessna’s Pipers etc the percentage must be close to zero.
The question made me go back in time. I hired for the first 10 years. No GPS, no A/P and often pretty marginal VMC where I lived. I wondered how I managed as a newbie. Would not do it today! I guess it’s all about dealing with whatever is available, and being younger and feeling more invincible
The big issue is to find a shop that can actually repair and/or troubleshoot these things, even modern units. In our club we are having constant issues with a GFC500 installed in a C182 and the shop (one of the most reputable in SoCal) cannot find the problem, much less fix it.
IME age of the equipment mostly comes into play when you’re dealing with really old units where the human knowledge base has retired and the new guys simply don’t know the tricks.
My KFC200 has random gremlins in alt-hold mode, works otherwise.
