A new motherboard isn't required if you want to upgrade and already have a 500-series motherboard. Owners of 400-series motherboards will also be able to upgrade but will need to wait a bit longer for BIOS versions, these aren't expected until early 2021.
Perhaps the biggest negative is that AMD is no longer the budget king. The company's renewed strength allows it to pick higher price points for its products, which means you'll have to get used to somewhat higher prices.
VideoCardz has an overview article that covers some statements made by AMD Director of Technical Marketing, Robert Hallock. On social media, Hallock answered a bunch of questions about the lack of power plans, DDR4 memory configurations, overclocking, and undervolting. Hallock stated a new undervolting functionality is coming via an upcoming AGESA update:
But Robert, where is undervolting? Answer: IT’S GONE. j/k. Temporarily gone for 5000 series. It’ll be back in an upcoming AGESA with new functionality. It’s gonna be HYPE. Y’aint never seen undervolting quite like this.
“CAN YOU TELL ME MORE?” No. ¯_(?)_/¯