NextJS is the perfect framework for small to medium size apps (provided that you use Vercel) as it allows you to create custom API routes that interact with other APIs or even databases without having to deploy a separate service.
The other advantage that NextJS has over Gatsby is its ability to be natively server-side rendered or if you enable it to implement Incremental Server-Side Rendering it makes the ideal blogging platform.
For both of the two projects that I've made with NextJS, they've both used next-auth which is a single `yarn add` away and a single config file where you can enable a variety of different OAuth or other authentication options with ease. It makes authentication easy for the uninitiated and has perfect integration with any NextJS adapt.
Vercel is the missing half of NextJS, if you weren't to use Vercel to deploy your NextJS projects it would be nigh impossible to get the serverless code to work properly. Though other Static Site Providers provide a similar service because NextJS was made by Vercel it has deep integration that makes it easy to use and write code for.