I worked at SimSpace, we had a million lines of Haskell written in house. It was wonderful! It was broken up into 150-175 packages with a surprisingly shallow dependency tree, making compile times decent.

It helped that our large application was a bunch of smaller pieces that coordinated through PostgreSQL.

We had three architects who spent their time finding near future problems and making sure they didn't happen.

I've had Haskell jobs with smaller and worse codebases. I think bad code can be created in any language.