When you're going to deploy a React application, you've typically used a tool like Webpack to compile it down to a set of static files. Once you've done that, deploying it via nginx will be smaller and faster than an Express server, and since you don't have the entire Node interpreter involved, there are fewer parts involved to potentially have security issues.

In fact, even if the rest of your application is in containers, it can be beneficial to host the front-end somewhere external (if you're otherwise deploying to AWS, for example, put the built front-end into an S3 bucket). This can simplify the deployment mechanics a little bit, and with Webpack's file hashing, you can keep older versions of the code for browsers that haven't reloaded the page recently. You still probably need the nginx proxy (to serve a /index.html page, to proxy the back-end services) but you don't necessarily need to redeploy it when the front-end code changes.

The only real advantage to an Express-based deployment setup is being able to live-reload your code in production. That would come with a high risk of human error, though: make an accidental typo and save the file, and end users see an error page. I'd avoid this path.