I think the rest of us who interact with it don't really fully understand what's going on. We've basically traded one set of scripts only a few folks understood (bash, perl) for Terraform in this case.
I totally see the benefits, but sometimes I feel like we try to force everything into it, even when it might not be the best fit.
I tend to preach separating things out and keeping things small. Why do you need a huge TF project to manage 15 services and have weird conditional parts when you can have 15 smaller projects that can deviate from a boilerplate template where it's sensible? I get that running "terraform apply" in one place makes the ops team's life easier, but the mess it creates in the actual TF isn't worthwhile IMO. And god forbid you use TF for your infrastructure and deploy using something that works more appropriately.
Solving Imaginary Problems At Scale is a thing. Monolithic TF – or at least one per env – will take you extremely far, and is easy to immediately understand. When it comes time to break out of it, don’t go overboard.