Bit Tech reports about possible compatibility issues with Intel 300 chipset based motherboards and the new 9th Gen Core CPUs. The problem is boards with older BIOS versions will not even boot to BIOS when you use one of these new processors, making it impossible to upgrade the BIOS if you do not have access to an older CPU. The site says Gigabyte offers an upgrade service for users that encounter issues, but other motherboard makers are less keen to help you.
What this does mean, though, is that for Asus and MSI at least, there appears to be a compatibility lottery going on if you pair one of these first-generation 300-series chipset boards with one of the new 9th Gen CPUs, such as the Core i5-9400. Despite both companies claiming these boards will be compatible, there is clearly old stock out there with BIOS versions that are completely incompatible with these new CPUs. So, my advice to you is to buy Gigabyte, since its board worked, at least for me, plus in the UK it will apparently update your board to a compatible BIOS for free if it doesn't. If you go with someone else, be prepared to click that RMA button.