What is the difference between Android's built-in Data Saver and the Google app Datally?

Data Saver prevents background apps from using mobile data.

Datally blocks apps from using mobile data but each can be unblocked easily.

Although these can both be enabled at the same time so far I've tended to use Datally exclusively because blocking apps from using mobile data is a stronger restriction than blocking background data access.

With Data Saver the currently active app can use mobile data.

I'd like to say that this was obvious to me but I initially switched because it is easier to understand what Datally is doing. It's binary - mobile data is either on or off for apps.

So far the only complaint I have, a minor one, is that it blocks some apps/services that shouldn't be blocked (e.g., the Phone app? Really? I prefer text as much as the next guy but blocking the Phone app on a smartphone seems aggressive).