A paraphrasing of the discussion:
q: "What issues did you have with Haskell in production?"
a: "PostgREST doesn't provide binaries and building it is hard"
q: "Actually PostgREST binaries are provided for multiple platforms." [1]
a: "The real reason is we don't have Haskell developers here." Case closed.
From personal experience PostgREST is high quality work; a set it and forget it service that doesn't need babysitting. Bugs are typically about missing capabilities, as opposed to failures. "Low defect" Haskell that does what it says on the tin.
[1] I don't know exactly when binaries first appeared but they were available as of April 2016, about seven months before the first prest commit.