Here's my understanding of the situation:
* a network interface can either be active/started -- i.e., is in the "up" state, without necessarily having a network address -- or connected -- i.e., having an address; and
* a system can have multiple network interfaces.
I interpret "-s" to mean "all interfaces are active but do not necessarily have an address or a default route". This means that NM will return success once each interface is activated, but does not actually require that the system can reach the outside world.
Without "-s" it means "there is at least one interface with an address and default route".
The problem with the former is that it can return before your system can reach the outside world (e.g., interface is up but doesn't have a DHCP-assigned address). The problem with the latter is that if your system has multiple interfaces, as soon as *one* of them has an address and a route, NM says all is well and continues, *even if that interface can't reach the outside world*. I ran into the latter when my Ceton TV capture card -- which used a virtual network interface -- would come up and NM would say, "this 192.168.200.1 device has an address and a route, I'm done!" and continue along before my actual network interface got an address. This caused issues with various services, and -- if you check the mythtv-users archives -- you'll see that the systemd people's response was "working as intended, that's a bug in mythtv and the other pieces of software which don't adapt to network interfaces which appear and disappear randomly."