Ok so I know according to Wordpress' own article on this that the core gets security auto updates since 3.7. But my question is does that include all security fixes? I.e. if I have a site that does get auto updates say version 4.0 are there any security benefits to updating to 4.6?
Yes there are a lot of benefits.
There are 2 types of release:
- Major WordPress releases, e.g. 4.0, 4.1, 4.2
- Minor releases e.g. 4.1.1, 4.1.2, etc
Normally if a major security hole is discovered, it's backported to the older major versions, usually the previous 3 or so ( I forget exactly how many ).
Keep in mind that this is only for security fixes. If you want to avoid other bugs, and ensure that you're not left behind when a new release is out, update to the latest major version.
Also keep in mind that a lot of security changes won't take the form of hotfixes, only urgent holes are backported. For example, newer versions of WordPress have much better handling of HTTPS sites, as well as improved password behaviour. These kinds of changes improve security, but aren't backported, as they're security features, not security fixes.
If you're looking for a simple one size fits all answer, always stay on the latest major version and test your upgrades.
There aren't many situations when that isn't the best case, and they're all super localised or related to code that didn't work with the upgraded version