As far as mode versus SHM armed states, it’s easy to get confused. The following thread should help:
As far as whether SHM itself is useful, different things work for different people. It has two advantages that you can’t get just from core. First, it can be linked to the scout monitoring system. And second, some parts of it run locally (although you can’t arm/disarm locally 
).
https://support.smartthings.com/hc/en-us/articles/209979766-Local-processing
But if you did want to have a valve connector turn off the water valve when a leak is detected even if the Internet is out, you can do that with SHM and you cannot do that with core because core does not run locally.
The lack of entry/exit delays is very strange for a security system, no question, so there’s not much to say about that except that there are obviously advantages to using core as well.
So it just comes down to a case-by-case evaluation as to which you use.