There has been some talk at work about making it a department-wide policy of prohibiting the use of unsafePerformIO and its ilk. Personally, I don't really mind as I've always maintained that if I found myself wanting to use it, it usually meant that I need to rethink my approach.
Does this restriction sound reasonable? I seem to remember reading somewhere that it was included mainly for FFI, but I can't remember where I read that at the moment.
edit: Ok, that's my fault. It wouldn't be restricted where it's reasonably needed, ie. FFI. The point of the policy is more to discourage laziness and code smells.
A lot of core libraries like ByteString use unsafePerformIO under the hood, for example to customize memory allocation.
When you use such a library, you're trusting that the library author has proven the referential transparency of their exported API, and that any necessary preconditions for the user are documented. Rather than a blanket ban, your department should establish a policy and a review process for making similar assurances internally.