In my opinion, Terraform's big issue is that it was born as a declarative tool for managing infrastructure. Large configurations (IMO) necessarily ossify, because you don't have an imperative language that makes small progressive changes toleratable - it's a giant interrelated lump.
What's worse, when it grows, one needs to split it in different configurations, and one loses referential safety (resources will needed to be linked dynamically).
A, say, Chef project of equivalent size, can be changed with more ease, even if it's in a way, even less safe, because you have the flexibilty of a programming language (of course, configuration management frameworks like that have of different set of problems).
I'm really puzzled by the design choice of a declarative language. Having experience with configuration management, it's obvious to me that a declarative language is insufficient and destined to implode (and make projects implode). Look at the iterative constructs, for example, or the fact that some entities like modules have taken a long time to be more first class citizens (for example, we're stuck with old-style modules that are hard to migrate).