I get the impression (I have no personal experience) that many of the X370, B350 and B320 motherboards were pretty bad. Something to do with the board makers not being sure AMD had a serious contender with Ryzen, and thus being unwilling to invest a lot of resources in developing solid boards.
Actually. I lied. I built my stepson a system with a Ryzen 5 1600x and an MSI B350 Tomahawk (Microcenter Bundle Special) in December 2017, so I do have a little bit of experience.
The Tomahawk doesn't seem like a bad little board.
So, here is the truth behind that. There are a few reasons why so many AM4 motherboards are kind of
"bad". It comes down to a wide range of things. First, Intel moved up Z270's launch date to interfere with AMD's Ryzen 1000 series and AM4
(X370) launch. This strapped design teams and motherboard vendors for time. Naturally, they concentrated on the Z270 launch which was first due to the new time table.
Secondly, expanding on the earlier point, your right. Motherboard manufacturers had to choose where to hedge their bets and they went with Intel. Resources and money were not spent on developing as many AM4 models, or building them to higher standards. If they didn't sell and they cost allot, the motherboard makers would be out more money. The boards aren't necessarily ****ty, but motherboard partners built less expensive motherboards as they can always sell budget offerings and had successfully sold AM3/AM3+ in the past even though Bulldozer was basically a huge steaming turd. Sometimes you need to build a computer and have no money, so you compromise. Sometimes your building for friends or relatives and either they have no money, or you don't want to spend a ton of money on a computer for someone else. Whatever the reason, loss leaders can always sell.
That's why you see so many bottom dollar B350 boards with questionable VRM's. Also, AMD doesn't have the control over the motherboard partners that Intel does. There was no reason at the time to build AM4 boards at that price point that were capable of delivering 200 amps to a 16c/32t CPU. The only reason why those boards will work at all, is because AMD managed to keep the Ryzen 3000 series within that same 105w TDP as the earlier chips. So at stock speeds, you should be OK on most boards. Its up to the motherboard makers to update the legacy boards with a Ryzen 3000 series compatible BIOS. They may choose not to do so on borderline designs. We'll have to see.
The same thinking led a different direction with Threadripper. Motherboard makers weren't sure that Threadripper / X399 would succeed, and that platform couldn't be built cheaply. Motherboard makers decided to build good designs, but restrict their design teams to one or two high end models and take a wait and see approach. Obviously, AM4 and TR4 were far more successful than initially anticipated and we have a ton of B350, X470, B450 and X399 options out there now.
X570 is a different animal for a multitude of reasons. AMD now has more clout with the manufacturers to say: "We want you to build X570 motherboards as premium option." The technology behind it also has more strict requirements and for enthusiasts, better VRM's are required. Basically the VRM's are beefy on all X570 motherboards and the PCB's are far more expensive. AMD opted to go with newer more advanced materials than increase PCB thickness using conventional materials. So there is added cost there that will come down eventually. The actual X570 chipset itself is more expensive as well. It's derived from the I/O die of the Ryzen 3000 series CPU itself. It's a power hungry 15w part that needs more power itself. I'd bet most if not all X570's use 32MB BIOS chips too. So the costs keep going up.
Ryzen is obviously selling well and AMD has serious momentum right now. Hence the shake up with X570 over earlier and more budget conscious X470 designs.