I don't know much about Asymmetric Dual Channel, I wasn't even aware of its existence until now. But keep in mind that when pairing different memory sticks, the whole memory will work at the slowest stick's speed. So if you pair the 16 GB 2666 Mhz stick with a 4 GB 2133 Mhz stick, both will work at 2133 Mhz. Same goes for the timings.

As for the timings, the lower, the better. Usually, the DDR4 has a CAS Latency (CL) between 10 CL and 19 CL, and these go the opposite with the frequency. Lower frequency 2133-2400 Mhz -> 10-14 CL, higher frequency 4000+ Mhz -> 19 CL.

Personally I won't bother with memory which has 17+ CL while the frequency is less than 3466 Mhz. From what I've seen, most 2666-3200 Mhz memory has 16 CL, while faster ones have 14-15 CL. For 2400 Mhz, I consider 16 CL slow memory, and for 2133 Mhz, 15 CL is slow as well.

A better link for your motherboard is the manufacturer's site. I don't see there anything specified about Asymmetric Dual Channel. My guess is that anyone who is interested in performance won't even consider pairing different types of RAM modules, so probably manufacturers don't bother too much with Asymmetric Dual Channel either, just my 2 cents. :)

Edit:
In the link above, on the manufacturer's site, you have at Support > Memory QVL (Raven Ridge) the list of memory tested by them for Ryzen APUs.

IIRC x-channel is used almost exclusively with laptop type motherboards where they want to sell memory upgrades and there are significant customisation and space limitations so something like an 8+4 configuration* makes some sense. ITX would be the closest desktop form factor to a laptop one so there was some possibility but yeah, you're pretty much expected to just buy a paired kit. And even if it were a feature on a desktop board- which I'd very strongly doubt unless added to some baseline specifications- I'd be equally surprised if the documentation bothered mentioning it.

Personally I'd be selling either the 4GB or the 16GB (or both) and getting a paired set. Losing dual channel won't kill performance; but dual channel is 'free' performance if you have it.

*1+2 configuration was the 'original' use, iirc, as that made sense in 32bit systems where 2+2 would run into the 32 bit limit anyway.

Thanks for the clarification. It makes perfect sense for laptops.

I had no idea about the asymmetric dual channel thing either until I read it from you and Phasmid and the only thing I know about CAS latency is what you told me. Thank you guys by the way, I really appreciate it. I would seriously mark both of you guys as solving this but I just went with the "underdog" rule.

Before, I thought as long as the RAM is the same speed, manufacturer and model, I'm good to go but then again even Ryzen itself was a different beast because before this I would've just opted for memory that was good ol' 2133 mhz but now, I already returned the 16gb 2666 mhz so I might as well.

Either way it seems like I should really just opt in for the 32gb (double 16gb) option and call it a day. I know how to put computers together and the simple concept of putting 16 + 4 together just got way too complicated. I do plan on running games on the ultra settings, so this may help more than harm.

wow, loads of good information about ryzen + mem configs here ... thanks and pls keep it coming :D

Thank you. That's why I started this and before this I had no how my APU was different from the first Ryzens that came out and now I have a really good idea. I have overall decided what I'm going for in my configuration but feel free to ask about different configurations. I'm unable to test any different configurations out but the general advice on this seems to be that dual channel seems ideal and it should be at least 3000mhz bus speed in order to keep everything up to speed.

Bump, but there does now seem to be an answer to whether asymmetric dual channel is supported by (Ry)Zen processors- and the answer is actually yes.

"Yes, you can run dual channel configurations with mixed memory sizes on "Zen." We [AMD] don't have a name for it. I cannot speak for older platforms, as I did not work in the CPU business during that time." Swiped from reddit.