Hi Alessandro,
The one point that implementation DLL have that no any other customization have is it is residential,
Your UDO form (If you have registered it) can always run with/without your addon running.
If you rely the form validation on your addon then you will get junk data inside because user can add any data into it without validation (remember the form can be open without addon running).
On the other hand, the DLL is always running with the SBO. Your Business Logic will always be enforced.
Agree with you that it looks like it is now left behind by SAP. My conclusion is that the B1UDOApi.lib was only built for 32bit.
Since the 64bit version of SBO, if they intend to support this, there are few things that they need to change. First thing come to mind is the registration of the this DLL it self. Currently it supports only one version in the table OUDO.
The fact that it is a C++ only and because of this barrier not many developer are leveraging this feature, make it now really obsolete 
If you were to ask me, I would actually prefer moving all the event handling into the DLL if possible, instead of the current implementation. The current implementation only supports handing of OnAdd, OnUpdate, OnCancel, OnDelete and a custom invoke method.
I would just leave this thread open and see if anybody from SAP would confirm on this.
Regards
Edy