Rails does plenty of "make life easier for the programmer" things that I would expect to increase the risk of security issues. Do you have those kind of problems for e.g. Haskell?
There are very fundamental connections between strong typing, program verification, and proofs.
http://en.wikipedia.org/wiki/Curry-Howard_correspondence
Thus, the argument that Haskell probably has the same, is simply false.
There are large web platforms in Haskell. Yesod is probably the largest eco-system. It is clearly not as well used as RoR, but anyone can dig through large amounts of code to try to find these bugs.
What Haskell has that everyone else has are bugs/misunderstandings in how protocols are implemented. Sometimes there can be fundamental bugs in the run-time-system. However, large classes of bugs are fundamentally less likely to appear than in less safe languages.
For example, here, if the guarantee of functional programming is that a given input leads to a given output and has no memory side effects, then your attack surface area is a lot, lot smaller.