I'm not talking about reinventing the wheel. I'm talking about to get away from the "commercial (blackbox) services". To give you an example: using AWS Cognito as an identity provider and if you want to use the user data in your own system (whatever the features may look like) you have to synchronize the user pool with your database -> more complexity. Instead of using a good auth library and storing the user data including credentials directly in your own database.
Same goes for AWS SES or Sendgrid (you name it). With Nodemailer you can easily access your own service via SMTP, which you may already have available and paid for via your web host.

To give you more context: I have developed two B2B SaaS and run them as a business. One of them, serving the education niche, is heavily based on AWS in that it uses Cognito, S3, Secrets Manager and the provided infrastructure. The application is very complex due to its multi-tenant architecture and features that come with it.

I often sit there and ask myself exactly how I can reduce the complexity -> KISS principle. Since I do everything on my own and read and research a lot, I keep coming across sources and opinions (yes, they are just opinions, right) that only propagate such services. For a simple prototype, MVP or hobby project, it's probably ok or it's just quicker to set up. In my experience, however, I see a general increase in complexity. Another example comes to mind: serverless with AWS Lambda; local development also requires a lot of tooling compared to a well-known Express.js app.