Windows 10 Professional OEM vs Retail
A few key things that the Terms and Conditions for an OEM key states that:
OEM versions do not offer any free Microsoft direct support from Microsoft support personnel
OEM licenses are tied to the very first computer you install and activate it on
OEM versions allow all hardware upgrades except for an upgrade to a different model motherboard
OEM versions cannot be used to directly upgrade from an older Windows operating system
With any key, I have never used services from option 1. However, I have bypassed option 2. and 3. I just had to call a number, talk to someone and I had to enter some sort of validation. It's been a while since I've had to do that, but the process wasn't too complicated. They did ask why I replaced the board, and each time it was due to the board failing. All of the times I did this, I worked at a PC repair shop. I'm not sure what they would have said if it was just a board to upgrade to something new.
Somewhere in the Terms and Conditions, there is a limit to how many times you can reactivate an OEM key. I don't know what the magic number is.
As far as option 4. goes, I think the only time that worked well without having to get a new key is when Microsoft was basically letting anyone and everyone upgrade to Windows 10.
I myself got the Windows 7 retail version when I built my PC several years ago so I wouldn't have to worry about reactivating if I changed hardware. With that one purchase. While on Windows 7, I had to RMA a motherboard once. I also took advantage of upgrading to Windows 10. I've upgraded my CPU and motherboard since then and had no problem with reactivation.
To me, it was worth the extra bit of money to avoid some potential hassle, or possibly being denied a reactivation.
Here's a link with some easy to read OEM Software Licensing: Rules & Restrictions