AlphaCT: replying to this and your private inquiries:
I've been told, off the record, that the cause of most of the unable to fetch config issues is caused by the file system on the MX that stores the config, which is different than the file system that houses the OS, dismounts (why has not been shared with me), and 15.4X solves most if not all of these issues. "Unable to Fetch Config', in essence means that the config is unable to be updated between the local file system and the cloud. When support "does something on the back end", they are mounting the file system, which brings the MX back online, but does not solve the problem permanently. Meraki support has been hesitant to share these details, I suppose because if it got out to competitors or the public it would be a pretty big black eye; I get that, but there's also a need to know for those of us that sell and support Meraki gear; hard to say what the right choice is; I see both sides.
We've had no more Unable to Fetch Config since I upgraded all 145 of our customer's MX64s to 15.40 about 2 months ago, and we had 5 in pretty quick succession prior to that. I asked why this suddenly became an issue with us (all of ours had been running 14.53 for a long time prior to upgrading to 15.40) and they said it had been an issue in all 14.XX revs, and is fixed in all 15.XX; I suppose it could have been a coincidence we had 5 in about 5 weeks and none prior, but coincidences are rare, so I suspect there's something they're not telling us.
Earlier this week support automatically upgraded 62 of our 110 customer Organizations from 15.40 to 15.42. Why not the other 48 is apparently just random, according to support, and presumably they will get upgraded soon too.
There is no difference between 15.40 and 15.42 relating to the unable to fetch config issue, according to support.
The only problem we have had with 15.4X is a client VPN problem. There's a backend fix that is needed to get an MX with Client VPN to pass the Diffie-Hellman group and Cipher parts of the Trustwave PCI Compliance scans to pass. That fix is not working with 15.4X, so customers are currently able to use the Client VPN but not pass the Trustwave scans, so this is only an issue for customers that have to pass PCI Compliance scans AND need/want the Client VPN AND that you move to 15.42.
We have not had any site to site VPN problems related to the unable to Fetch Config issue, 15.4X or anything else. The resiliency, stability and ease of the Meraki site to site VPNs is amazing (especially for anyone that's been exposed to how cumbersome and finicky Cisco site to site VPNs are). I saw you said that was someone else that mentioned that, but just figured I'd clarify for anyone that comes across this post.
So, while it makes me very nervous to be running 15.4X Beta on 145 customer MX64s, so far so good (other than the Client VPN issue), knock on wood.
John