If you have an older ath9k device (e.g. TP-Link TL-WR1043ND v1), there have been problems with the radio (which should be fixed now). The more people can test it, the more we're sure this is fixed before release.
I think that there was a two day delay in the image build as the tagging script failed and needed a fix.
"do not move" means that the tag has a permanent commit hash (instead of the moving HEAD of the 17.01 branch), and the tag also sets a fixed commit hash for each of the feed definitions. In practice this means when you checkout the rc1 tag later, all feeds like LuCI, packages, routing etc. are automatically set to the fixed commit in the feed, instead of HEAD.
It has always been a challenge to explain to users who want to build the exact "release CC15.05.1" etc. that it is not enough to check the main branch sources from that release date, but also the feeds need to be from the same date.
Great that Jo automated the fixing of it for LEDE.
Release candidates are primarily meant to find and ideally fix bugs before the final release, I wouldn't expect there to be changes in regards to the installed package set or significant differences in regard to flash or RAM usage/ utilization. Devices with just 4 MB flash/ 32 MB RAM are seriously challenged when it comes to running current software, imagebuilder might help you to squeeze in your missing packages this time - or it might still remain to be too large, but it's worth testing.