I have used Terraform, Puppet, Helm, and Ansible (although that's not strictly declarative), and all of them ran into problems in real-world use cases that needed common imperative language features to solve.

Not only does grafting this functionality onto a language after-the-fact inevitably result in a usability nightmare, it also gets in the way of enabling developer self-service for these tools.

When a developer used to the features and functionality of full-featured language sees something ridiculous like Terraform's `count` parameter being overloaded as a conditional (because Terraform's HCL wasn't designed with conditional logic support, even though every tool in this class has always needed it), they go JoePesciWhatTheFuckIsThisPieceOfShit.mp4 at it, and just kick it over to Ops (or whoever gets saddled with grunt work) to deal with.

I'm seeing the team I'm working with going down that same road with Helm right now. It's just layers of templating YAML, and in addition to looking completely ugly and having no real support for introspection (so in order to see what the Helm chart actually does, you essentially have to compile it first), it has such a steep learning curve that no one other than the person that come up with this approach wants to even touch it, even though enabling developer self-service was an explicit goal of our Kubernetes efforts. It's absolutely maddening.