I had the same dual monitor problems even after migrating to Nouveau. It's Plasma that's broken.
I'll acknowledge it could be something about my hardware setup. However, the business-grade Dell Latitude E6530 works beautifully under 14.04. That says broken software, or a changed design that's immature.
Also, Plymouth password input for decryption of full encrypted disks is broken. Luckily, as a long time user I knew how to recognize what was happening and get around it but a novice would rightfully think their system was inaccessible. It's something to do with USB HID support broken in the ramdisk boot images but I couldn't fix it. And I shouldn't have to in an LTS release. Just one more little thing that says 16.04 is overly-ambitious and immature.Edit
: I suspect Good things will come of the changes being introduced in 16.04, but like the first introduction of Plasma 4, this is a release that's best for most people to skip over until things settle and mature.