The April 21 build seemingly still does not work on flagship Lumia Icon (929) or Lumia 930 devices, due to screen size/scaling issues. It also still doesn't yet support Windows Phones from any of Microsoft's phone OEMs.
The Windows 10 Mobile build Microsoft released a couple of weeks ago included a number of new features, including Microsoft's "Spartan" browser and new test builds of Outlook Mail, Calendar, Phone, Maps, Messaging and People apps. Even for a "preview," that test build was very unfinished and hard to use, a number of testers said.
Last week, company officials said they weren't sure if they'd release the test versions of these apps as part of a new Windows 10 Mobile preview, or if they'd make them available separately through the Windows 10 Store.
AMD's CEO Lisa Su revealed last week that Microsoft is planning to launch Windows 10 in late July 2015. From what I've heard, this launch will include not just Windows 10 for PCs, laptops and tablets, but also Windows 10 Mobile for phones. Some, if not all, new devices supporting Windows 10 could lag the launch by a number of months.