When will it be safe to sysupgrade to these new images? Has anyone actually tried them? Or is there some reason we should remain with the snapshot? Should we wait till the table of hardware changes before sysupgrading?
The table of hardware will be updated once a stable release supporting your device is available or that is my understanding so far. The v19.07.0-rc1 is going to be the first stable version for your device once it gets released as a stable version and rc tag will be removed. Snapshot version will be more buggy than the v19.07.0-rc1 because there's no importance in snapshots on fixing bugs unless they are critical but the above version is being tested by many people to see if it's suitable to be released as a stable version. You can flash it if you want to provide any feedback for your device because there may be a bug for your device that haven't been identified before or something like that and if it's gets found it will be fixed in the stable version as soon as it gets released.
This is not necessarily true. Virtually all development work happens on master, including the vast majority if bug fixes. 19.07.0 reflects where things were in early June. There have been months' of development work since then.
The release provides the convenience of pre-installed LuCI, and, when actually release, a stable archive of packages.