Because it is hard to know the implementation details of every search engine and what reasons different markup could result in a subtle difference in ranking, I

Because it is hard to know the implementation details of every search engine and what reasons different markup could result in a subtle difference in ranking, I think the best way of answering this question is looking at the markup differences between HTML produced via ReactDOM.renderToString() vs. some other solution. In versions of React prior to 16, some extra attributes were added to the markup, but besides that it looked basically identical to what you'd get using any other method. Now there is even less difference, so it's hard to imagine React SSR having significantly different SEO characteristics. But one caveat is that if you need to hydrate serverside-rendered React markup for clientside use, that will add at least some small performance penalty. And you will also want to pass in props data that matches as closely as possible between the client and server, which means that your base page will need to include some serialized version of the data that should be passed in as props to the component(s) you serverside-rendered and want to hydrate. If this is a large chunk of data, there could be negative SEO impact if it is placed above meaningful content in the page or if the time to download it delays complete load of the page.