The problem with the left AF sensor on the Nikon D800/D800E is well known. I got the impression that is quite frequent. Personally I don't use neither Nikon nor Canon and I don't shoot weddings. I'm essentially a landscape shooter. Regarding AF, I simply don't trust it. My experience on Sony is that AF is about perfect at f/8 but often focuses on the wrong stuff. So I use center AF and focus recompose. If I can I use live view at 11x magnification.
You are absolutely right that there are issues with camera electronics, even if I must say I have seen little of that on my recent cameras. I had two Minolta DSLRs and five Sony DSLRs. I have not seen any electronics glitch. The main reason for me to upgrade was live view and the need of having a backup camera. I have kept three of the latest camera, two full frames I normally use and a high resolving APS-C for telephoto work and walk around. In addition I have had five long zoom cameras Minolta 7i, A1 and A2, a Panasonic and a Canon. One of the A2-s had a repair for a minor glitch (macro switch not working). That was about it. In addition I had bayonet replaced on an old 80-200/2.8 APO lens, returned an obviously bad Tamron sample, sent an obviously broken Minolta lens to service, just to have it sent back to me saying it was within specs...
So that was about it, 40+ years of experience. I also had a problem Minolta in film times, it was called the XD7. Leica was selling the same camera chassis as Leica R4 and had also a lot of problems. I guess Minolta went from mechanical to electronic and did a poor work on the new technology.
I looked at repair statistics on Lens Rentals. They don't report on Nikon D800/D800E left sensor issue, as they say they don't have enough statistical data. Both the D800/D800E and the Canon 5DIII are among the most frequently failed cameras. Nikon has problems with broken battery doors and on the canon the most frequent problem seems to be bent CF-pins. Lens rentals indicated that most other DSLRs are not far from getting on the list.
Regarding repairs Tamron repaired all their stuff with no cost. Sigma was shining with fast repairs at reasonable cost.
Sigma is interesting, some of their new lenses seem to be excellent. A Swedish montly (Foto) has done lens testing at Hasselblad (yes Hasselblad folks do their testing at Hasselblads labs). They compiled a list of then best lenses in 40 years of testing and one of the Sigma lenses made it to the list. The author of the list said that all Sigma macros would match any of the Leica lenses.
Here is the list (in case someone is interested): http://tidningenfoto.se/de-skarpaste-objektiven-fotos-tio-i-topp-lista/
I'm in no way a Sigma fan, but it seems they are moving into high quality lenses.
I find it pretty interesting this never gets a mention. I owned a Nikon D800 that certainly has this issue and my D800e does as well, not as bad but still a issuehttps://www.google.com/search?q=nikon+d800+left+af+problem&ie=UTF-8&oe=UTF-8&hl=en&client=safari
And Canon also not without issues.https://www.google.com/search?q=canon+5d+mark+iii+issues&ie=UTF-8&oe=UTF-8&hl=en&client=safari
Now I agree this seems to happen more than any of us care to see and that goes for any OEM. I did not post Leica, Sony , Hassy and others but there always seems to be some bug or issue that crops up. Now as far as Phase yes I agree the DF had a power issue and it affected a good number of users even myself it happened to me in the beginning but like anything else you find workarounds and through a lot of folks trying diffrent things a switch in batteries helped a great deal, than like anyone else in the photo world a firmware fix addressed it as well. For most of us these things worked very well and some users maybe many never had any issues. What helped a lot of used was the ability to work with there dealers for any help needed in getting to a fix, repair and even replacements. Buy used and frankly your on your own just like buying a used Nikon or Canon. But dealers in 35mm are not like MF dealers at all and this needs to be clear. We had a workshop attendee on location have a complete DF failure , one phone call and a new body was in his hands by 10 am. Lets be really honest here shit happens and Murphy's law is ever present and I don't care what name is on the faceplate. My D800 failed twice for no reason. Pulled battery and turned it back on just like when I had my Phase DF. No difference same deal I could not shoot. I'm not dismissing the DF issue at all but on the same hand I'm not dismissing the left AF issue with my Nikons.
Sorry to say but with electronics this crap just happens , do you know a PC that never needed a reboot. Even the Mac diehards can't say they never had reboot or see the grey screen of death. I know I have. LOL
Unfortunately and I even hate to say this but the truth is with electronics this is becoming the norm and not the exception. Folks this is digital it has a lot of pluses for us but we all know it has its downside as well. It's tougher to shoot , you now are the processing engine and frankly maybe more work than we shot film all for the convienance of instant results. Good , bad or indifferent this is the digital age and this is our norm. The real trick or hope as users is these OEMs recognize there issues and make a effort to correct it, repair it, upgrade it or replace it. First they need to acknowledge it and start working on it and if they don't than they would be what we would call incompetent I would not consider any of these companies incompetent as they all took some measures to make things right. My Leica M8 two of them to be exact spent months and I mean months in Solms trying to figure out the sudden death syndrome they had. Luckily they gave me loaners just imagine as a Pro losing two bodies without backups for months on end . Not a good situation and one major reason when I went to MF from the M8 in full capacity as a replacement I went through a dealer to do that. I value there service as they have provided useful help over the years in MF and also a close friendship which leads to a relationship that you can count on when you lens, back or body decide it does not want to go to work that day . Like I said in the world of electronics you need to protect yourself from the new normal of products that are not exactly ready sometimes.
Now on to wedding work with MF. I don't do a lot of it but have used both together and each system on its own. Now lets face it 35mm is a little easier to deal with but seriously I never found MF that hard to deal with even under the pressure of shooting a gig in fast order. Honestly I never ran into a problem shooting MF where I mumbled to myself I wish I was shooting 35mm right now. Maybe it's me but I found shooting MF itself as a camera pretty easy. The biggest issue that always is a thought is more about DOF and making sure you can carry focus through a image. To me that is the biggest limitation if there is one and it happens in all types of shooting even in landscape work is carrying DOF. We all know the smaller the format the given extra DOF given the same aperture, its the one plus 35 has over MF. Now a lot of this depends on your thoughts on DOF to begin with and your expectations given the shooting gig. But I don't agree with you can't do weddings with MF. I never found there is much I can't do with any cam , its just a matter of your determination of getting something done and working within the limitations and knowing your work arounds and how your going to accomplish your task. Obviously there was a time when almost a huge population of wedding shooters shot MF film and they got along just fine. Does shooting MF digital pose maybe a change in how you go about your work over 35mm than sure I would agree, you certainly have to think more and different of how to get things done. Nothing wrong with thinking and working hard to get the shots you need, as a Pro doing weddings lets face it that is why your getting paid.