As to the belts falling off due to user error, I agree with that as well, and I would contend that the same is true for integrated weight system failures--when properly engaged, they won't fall out.
Except they do. There are lots of designs for IW pockets and some are excellent (Zeagle Ripcord is one). Others are not so good so I don't want to say all are bad, but in general I don't believe the reliability of a simple belt.
At one point I had 3 Mares IW jackets with the MRS 2 weight pockets and had multiple drops. At first I thought it was user error also (many of the drops were from my kids). Later I found several causes there were not related to failure to engage the release properly. After the first time, I started checking their pockets and they checked mine and we were super anal about it, but still continued to have a few drops until we nailed some a few other issues. All said and done, we had to replace 3 pockets over 2 years. Since then we are stable.
2X I had the pouches drop when re-boarding a boat. They worked fine for an hour long dive so they must be been inserted properly, but snagged on the ladder.
2X I traced drops to putting other things in the adjoining pocket (safety sausage). When the pocket was loaded to capacity and nothing in the pocket, it held. Add another lump of gear and the retention mechanism failed. It appears sometimes the extra gear would push on the IW pocket and release mid dive.
1X I found a strap pulled through which instantly released the pocket when you went horizontal. The MRS system has 2 straps. One is connected to the release handle and one carry's the load. The anchor is a tab pushed through a slot. It was pulled though by accident at some point. The pocket would load and snap into place as normal. Everything from the outside looked normal, but it would not hold a load as soon as you rotated from vertical to horizontal. Once I figured out what it was, it was a simple fix.
That's my personal experience. As I stated before, I have found plenty of pockets on the bottom at local training sites, probably close to 50lbs total. Sometimes I find the owner, sometimes I just add it to my collection (Hey the good news is I never have to buy lead again). Based on this I assume I an not the only one having some troubles with IW pockets.
Since then I don't trust the MRS pockets with a full load of lead needed for 7MM or dry suite. Its fine for warm water to holding 5-7lbs. Nothing more.