OPC UA servers combine the functionality of all earlier OPC specifications (DA, AE, HDA) and supports many new features. A UA server is therefore hugely more complex than current OPC DA servers.
The UA core implements a network database with nodes that describe items and item attributes. The nodes can be linked in a complex manner, allowing all kinds if information structures to be modeled. Network databases are difficult to implement therefore the more restricted relational databases are far more widely used.
The OPC Foundation implements a reference UA server and probably all UA server products will base on this reference implementation.

The server developer has to map the device data into the UA server network database. The complexities of the network database are less of an issue for the server developer than they are for the client developer. The server developer is free to choose a structure, either simple or elaborate. The client developer has to deal with different structures used in different servers.

Currently most OPC application use only the Data Access (OPC DA) functionality. This will probably not change because of OPC UA and therefore UA server SDKs could provide a DA specific mapping. This would bring the UA server development and configuration to the same level of complexity as the development of an OPC DA server based on a server toolkit product.

Advosol already offers an OPC UA Experimental Server with DA functionality. It bases on the Beta version of the OPC Foundation OPC UA SDK and is intended to give the opportunity to experiment with OPC UA at an early stage, before the UA specification is released and the UA SDK is fully implemented and tested.
The Advosol UA Server uses the same device handling plug-in .NET assembly as the Advosol OPC DA .NET Server Toolkit and the Advosol XML DA Server Toolkit. Users of these toolkits can experiment with OPC UA without having to develop any code. The user developed DA server plug-in .NET assemblies can be used with the UA server.