The problem with `concept` is that it doesn't work with `seq[C]`. If you're not aware of that, you can spend a huge amount of time debugging. After you're aware, you have to modify your API in potentially difficult ways.
Other than that, I think they're awesome. Not kidding. I love the syntax. For more discussion, see [https://forum.nim-lang.org/t/2999#25214](https://forum.nim-lang.org/t/2999#25214)
