I will add to this as time allows.
The 930 SPX had some issues to start with. Mossberg set themselves up for a perfect storm of customer service mishap and disappointment with the 930 SPX. It was very highly sought after when it was released. It was not a limited production gun, but it was not made in super large numbers; certainly not in sufficient numbers to meet the demand. They basically subcontracted it out with Choate doing extension components, LPA doing the sights, a third place installing those, and then the normal Mossberg production/assembly.
The sight assembly REALLY got screwed up when the barrels were not correctly placed in the jigs to have the sights attached to the barrel. QC screwed up because the cant wasn't caught before it left the factory. Because of the large number of screwed up sight towers, the Service Center was very quickly overwhelmed by the numbers of repairs. Add in that the Service Center further aggravated the situation by not taking care of customers, not having enough repair items, sending back new barrels that were canted as badly as the original barrels, and the limited number of staff having to deal with way more complaints than they were staffed to handle.
Another problem was that the front sight height could be off enough to cause POI issues. There is apparently enough of a tolerance issue between the sight, the receiver, the rail, the rear sight, and the barrel that some people had no problems with POI, while others had all kinds of problems with the sights maxed out. On top of this, OFM once again screwed themselves when they didn't put any info in the pubs that even told owners that the front sight was adjustable. To this day, I still talk to people that do not know that the front sight is adjustable. Who knows how many front sight problems weren't problems at all but were actully improperly adjusted.
There were also problems with mis-installing the screws that hold the rail on. There are two different lengths of screws, and sometimes the incorrect length was used. Some of them were overtorqued and stripped the receiver threads out and would literally fall out if the shotgun was inverted.
Reliability issues have generally been with brand new shotguns. The shotgun needs to be cleaned well before it is shot. Mossberg uses some heavy thick preservatives that can cause some problems. Best results with those that have had problems has been to run hotter loads through it to start, usually above 1200 fps. Some people never have problems. I haven't had any.
Some people have gotten the shotgun out of the box with a broken forearm retainer. Not a big deal, but should not be an issue with a new gun.
The feeding issues with the 930 SPX usually are usually due to two things: the mag extension problems and improperly sized FCG pins. The mag extension problems are typically a bent or dented or misaligned extension, a transition area problem between the extension nut and the mag tube that allows the follower to snag, or improper spring tension. The first one should be pretty easy to spot. The second one shows when you can feed the first round or two and then the follower hangs and can be popped loose, and the final one can be diagnosed by looking at the spring. If it is something like 30" or longer, it is too long and needs to be trimmed. This has put too much spring pressure on the shell latches, and the first round of a fully loaded mag tube will not release. This is being seen in people with new from the factory shotguns and also those that purchased just the extension kit from Choate. The spring should be I think in the area of 25-28" and some of them have been up to 40" in length.
If you shoot it for long sessions, you will notice the bolt slowing down after some time. When it does this, I squirt CLP in the open receiver and on the bolt body, then operate the bolt by hand a couple of times. It picks right back up.
I've been able to shoot even light loads in mine, and FedTac buck and slugs have been absolutely no problem.
The FTF issues hasn't been a major issue and isn't systematic like the other problems were.
Some 930s have what I would guess is a stacked tolerance issue in that there is just enough room in the bushings in the FCG, the FCG itself, and slightly undersized FCG pins allowing a lot of lateral movement in the reciever by the FCG. The Mesa sidesaddles have had a history of making functional 930s into non-functional 930s. It seems that this was due to small diameter connectors that allowed a little too much slop in the FCG of the 930, causing feeding problems. Mesa is supposed to be getting some new connectors to fix this issue, but I have heard that the new connectors or at least what were supposed to be the new connectors were having the same problem. Mesa is working on it, so hopefully for everyone that has one that will get worked out. I know there are a lot of happy Mesa users that have never had a problem with the connectors.
In addition to the QC and CS issues mentioned, Mossberg CS really messed up in how they handled a lot of the repairs. A lot of people called in to report the sight issue and try to get a replacement barrel. This was such a widely known issue that you could go to pretty much any board and read about canted front sights and low POI issues. How did Mossberg respond? The required the customer to pay for the shogun to be shipped to the service center. The whole shotgun. Many people reported this happening. In some cases, Mossberg agreed to prepay shipping and sent out the printed shipping forms. For a lot of people, the requirement to send the whole shotgun back, and especially on their own dime completely turned them off to Mossberg. I know of several people that sold their 930 SPX when they got it back from Eagle Pass. In addition to this problem, there were reports of a very, very slow turnaround. Some people have said that they were told that their shotgun had been sent back, only to have it never arrive because it didn't ever ship. The customer would call back and then be told that they were waiting on the replacement barrel.
If you get a newer production gun, you are not as likely to have the problems I mentioned above. The sight tower is on it's third version now, and they haven't had issues with the cant in a while. I haven't heard of a lot of the problems in a long time, so hopefully OFM has gotten their act together. I think you are much more likely to get a quality gun out of the box now than ever before. And even with the above issues lined up and explained, there are a very large number of users that never had any of the problems at all, ever. I didn't have any of them, and I know I'm not unique in that.