This was scala 2, so implicit resolution lookup was a big chunk of the problem. There's nothing at the call site that tells you what is happening. But even when it wasn't hidden in a companion object somewhere, it was still difficult because every import change had to be scrutinized as it could cause large changes in behavior (this caused a non-zero number of production issues).
They work well for anything you would use environment variables for, but a chunk of the ecosystem likes to use them for handlers (the signature being a Functor generally), which was painful