For what it's worth, Asus has provided excellent support for the PRIME X370-PRO motherboard. It was a bit of a gamble when I purchased a PowerSpec Ryzen system from Micro Center, but Asus has released regular BIOS updates, including support for higher-speed memory (e.g. I'm running 32 GB @ 3200MHz, Corsair Vengeance chips).
My only complaint would be the lack of detailed change logs, which are often only a single bullet point.
I have multiple Kaby Lake boards which haven't received any BIOS updates in > a year. It depends upon the model line and as far as I can tell they will not tell you how long support is before you buy one.
What are you expecting an updated BIOS to do for that platform? Kaby Lake was the last CPU line for that chipset series since Coffee Lake shuffled some pins around. So what are you wanting updated that Asus hasn't provided?
There are issues with the BIOS on on of the systems randomly changing the m.2 PCIe speed between 4x and 2x. It only exists in the most current version of the BIOS but doesn't happen on older version.
Their are also other issues that should be fixed but haven't.
