Not a deal breaker but Xmonad brings a large dependency (1GB+ is the price of Haskell) where most TWMs on Linux are in comparison laughably lightweight and blazing fast.
For me i3 strikes the sweetspot of dead simple config, incredible feature set, and performance. This after coming from Awesome WM where I found myself spending way too much time tweaking Lua script to get things pixel perfect, which you basically get out of the box for free with i3.