Lack of SendAs is one point that could result in "feature request" but stacking Z-admin or whatever else account management on top of Zentyal doesn't really make sense especially because Zentyal is not very flexible regarding LDAP back-end.
If you do need SendAs feature, then you should better go 100% Zarafa and drop Zentyal but, unfortunately, this will drop other Zentyal specific features
Looking at this from higher level, similar question occurs very often when it comes to design identity and account management. Unless you have build flexible LDAP backend and are able to segregate in a secure way which application is dealing with its own attribute subset, you often end up with only one single account management tool and merging or stacking with another one is not an option because of either conflicts or lack of LDAP attributes. To me, Zentyal is clearly on the "proprietary" side from this standpoint.
However, this is not a surprised and should not impact Windows users who are Zentyal's target