It is the nature of the Google services which microG tries to mimic that they can change. This can break microG compatibility for any given Google-dependent App at any given time, until microG catches up to the changes again, if at all possible.
This means that a Google-dependent App may work with microG at some point in time, and it may not work with microG at another point in time.
/e/'s Apps installer gives you convenient access to cost-free Google Play Store Apps, among other sources of Apps (Open Source Apps from F-Droid, for example).
It can’t possibly track whether a Google-dependent App happens to work with /e/ at the moment or not.
Perhaps users should know a little more about what they are dealing with.
Perhaps /e/ should drop the “you can still run your favourite apps” claim, because /e/ can not guarantee that for every single Google-dependent App for any given point in time, and elaborate on that to help users understand Google dependencies better and to enable them to use Google-dependent Apps aware of the limitations of a Google-free environment like /e/ … or ideally even to make the choice to get rid of their Google dependencies altogether.
Of course that’s a dilemma. “There’s a good chance you can still run your favourite Apps, but … it’s complicated, please read the extra page of text we prepared explaining that.” wouldn’t get as many users 
.