This won't help the "don't leave this overnight" issue, but if you want to see the security content of updates, subscribe to Apple's security announcement e-mail list. You'll probably get messages for products you're not interested in (e.g. iTunes for Windows even if you don't use Windows, watchOS even if you don't have an Apple Watch, &c.), and sometimes the content messages don't arrive until a week or two after the updates are available, but the messages do contain complete information on the vulnerabilities similar to what is, sooner or later, available in the notices linked to Apple's security updates Web page. I find it's helpful to have a record of what was in each patch.I haven't monitored the last couple of updates closely, but the whole process seems to be getting more and more complex, with long delays and multiple restarts for Macs running OS X 10.11 and macOS 10.13. I keep feeling like something is going wrong, but I can't pin down what. It does not help that Apple gives no indication what the updates are unless you check the App Store. I would like to see a verbose mode option to tell us what to expect and warnings of anything unusual, like "don't leave this to update overnight or it may freeze".
The broader issue of all the other content of the updates is something Apple has never addressed. Are caches being cleared (a process that takes a while)? Are filters being added to Gatekeeper? Are firmware patches being installed, and if so for what subsystem? Would it really give away some secret sauce if Apple told us (particularly those of us who administer machines for other people, e.g. customers or family members) that kind of thing?