My motherboard can run ECC DIMMS although they operate in non-ECC mode, any downsides?
Not much besides lower MT/s. I put my spare 32GB ECC kit in my Ryzen rig after one of my Supermicro X11 boards died, and it worked fine for almost 2 years now. One stick died not too long ago, but I bought the whole kit used for very-very cheap anyways, and 24GB is still more than enough for all of my current needs.
Once I save enough spare change for 5800X3D, I think I'll go with 64GB ECC kit. One of my suppliers has 16GB Crucial DDR4 ECC UDIMMs listed cheaper than mediocre non-ECC UDIMMs with equal clocks.
Depends on the board and their support.
With RDIMMs it's quite easy nowadays - it only works on server boards. ECC UDIMMs are a whole another story. It will work on most AM4 boards without issues, but it will not work on most consumer Intel boards. HEDT boards may also have gimped support(had quite a few X79 and X99 chinese boards that won't POST with ECC memory even though manufacturer/seller claims otherwise).
Basically my rules of thumb:
1) Avoid all of the chinese crap that's happily promoted amongst techtubers. They don't use it for longer than required for videos/projects, but as a daily driver it will fail faster than you can spell "refurbished excrement". No matter the tasty spec/price ratio, no matter the manufacturer claims, no matter how much stuff you can throw on it, it ain't worth it in the long run.
2) Used brand-name memory is a better buy than brand new no-name sticks. Lots of cheap chinese kits are also made by transplanting presumably working DRAM chips from defective used server/desktop memory. So, basically you are buying something that's just as risky, but also at some point was heated to at least 230C twice.
3) If you are using intel platforms - check the official docs. If ECC ain't there - it'll likely not work. That includes workstation/HEDT boards (with some very-very rare exceptions where ECC or RDIMM support was enabled post-factum).
