Alright, the latest patch note talks about "stability improvements". I've modified my Train map to use all 650 available items. I want to get a group of 4 people to test it to see if it's improved in any way. I'll record the tests and results. Each time we get a "stability improvement" from the updates, I want to repeat the test until finally, we can really call customs "stable".
It's still horribly broken FYI. I have a map on Awash with 249 and I can't ever get it to load with 12+ players. I host fairly large lobbies most nights and have a pretty good feel for exactly how many players the maps that I have can handle. Also, I've heard that players themselves are considered by the game to be objects, so if you want your map to load with 16 players then you should have 634 or less, but I have no way of confirming that.
In other words, Custom Games are still unstable as hell. Got it. Let's see what the next patch has in store for us...