The value for TFCDK was Developers don't have to learn another language, they can just continue to use existing language they already know.
Downsides are doing infrastructure in a programming language was always problematic unless developer was skilled at Ops which most who used TFCDK were not.
I ought to have phrased it I guess as "I don't agree with the value proposition", mainly because of the downside you point out. This seems superior to Pulumi, though, in that the abstraction is (was) at least owned by Hashicorp so there was less likelihood of it falling out of date and giving you footguns.
That might have been the promise but never the real value. As you say in practice the engineer needs to know ops & terraform along side their language of choice.
The real value of cdktf was more dynamic infrastructure provisioning while still having the plan / apply pattern.
Downsides are doing infrastructure in a programming language was always problematic unless developer was skilled at Ops which most who used TFCDK were not.