It seems the last thing you read about Ryzen was the 1800X launch reviews and have that forever ingrained on your brain as the indelible truth. Let me present you with some more current information, so you may offer more accurate advice regarding DDR4 and Ryzen.
AGESA 1.0.0.6 was released in May of last year to address memory compatibility and performance issues with the initial Ryzen launch SKUs.
In W1zzard's own words
from his review of DDR4 scaling on Ryzen - "We are happy to report that you can save some money by choosing a slower DDR4-2133 or DDR4-2666 memory, at least until DDR4-3200 or higher memory becomes more affordable.
You lose practically no performance to slower memory on the Ryzen platform, when averaged across our CPU tests. The fastest memory configuration in our bench, DDR4-3200 CL14, is about 3.1 percent faster than the slowest DDR4-2133 configuration. ...
The story repeats in our game-tests, where the biggest difference, all of 5.5 percent, takes place at the lowest resolution (1920 x 1080), while the difference is a meager 0.8 percent at 4K Ultra HD. ... It came as a bit of surprise to us that
memory speed didn't even affect performance of CPU-intensive tests, such as video-encoding in which large data streams are being pushed in and out of the main memory."
Furthering compatibility and performance is the hardware changes made to Zen+ in regards to the IMC, allowing the current generation of Ryzen CPUs to run a wider range of faster RAM. There is still a need to cherry pick for the ultra-fast (DDR4-3466+) but for 2666-3200 just about any off-the-shelf kit will do. I personally run a kit of Corsair DDR4-3200 that is
not rated for Ryzen compatibility (predates Ryzen, as a matter of fact) and yet still runs at the default 3200/CL16 XMP profile on a post-AGESA 1.0.0.6 UEFI.