This is exactly my experience. With any decent digital back for the 500, you will have to completely relearn your shooting technique. Forget depth of field, with digital there isn't any. So you may be alright for landscapes, still-life or whatever, but shooting candids or moving subjects will be impossible. With those kinds of subjects and MF digital, you have to have auto-focus. It's not a luxury, but a necessity. With film you can pre-focus and use the DOF scales to bracket your subject, but this approach simply will not work on an MF DB. The second point would be that a (relatively) cheap digital back for the 500 is really not worth messing with. If it goes wrong, it will cost way more to fix than it is worth. As someone else pointed out above, you would be better off spending the money on a decent scanner instead.
I will add another perspective to your statement. Not that I'm saying that you are wrong but I have a slightly different experience.
Don't know if this is a 500's exacerbate issue that's involved.
When I first hanged the Contax and focus manually, my rate of well-focussed frames was extremely low, as you point.
Then I started to examinate the files and realised that the manual errors where always similar and start to apply little corrections in the field.
Using a reduced range of focals, you finally get used of, let's call it a sort of 6th sense and your brain automatically "understand" the idiosyncracy of one particular gear.
What you point is also true in FF 35mm if you use manual focus, but it is magnified in MF.
I completly agree that there is a big difference between film age, digital does not forgive.
But if an AF can get the correct point, is that correct point exists and can be trained.
IMO, not contradicting your experience on that matter, I think that most of the time, (or we are in times when) taking this training into account and
accepting a learning curve is what is also missing.
Many times I just want to focus quickly on a part of the frame that I just feel more "creative" and if I had to relly on any AF, I just miss the shot on moving objects.
So, yes, manual focussing is more complicated in digiland, but like it or not you have to train this sort of 6th sense (don't know how to call it).
If not then you are just chained to the machine. Because let's say you focus first AF then reframe very quickly. You will have to apply anyway a little correction manually.
If you don't train yourself in manual focussing, the way you will apply this correction will be hazardous.
That's also the case with any dslr.
You can not relly on the viewfinder really, but you end to learn in what sense the viewfinder is fooling you.
I also use a lot MLU on field and that also makes differences.
Again, I'm not contradicting your point and your experience, but adding a "yes but" that IMO has to be taken into consideration.
All I can tell you is that having taken the time to deal with that (insisting on focussing manually despite the issues), my rate of
well focussed images has increased dramatically on moving subjects, and I feel more free to frame and choose my focus point by myself.
Edit: Mr Reichmann pointed in some articles that when it comes to the P65 sort of backs, forget about brain training for focussing. I've never tryied such a back and I suppose it is logical.
But he also stressed that this not relly only on the AF but all the elements on the chain: tripod, MLU, exposure, lightning...
It happens that the AF of manual focussing has been correct but one element of that list where missing.