Speaking as a user of XMonad for a few years now, I think the way tiled window managers tend to present more as toolkits for assembling your own tiled window manager rather than one that's just ready-to-go for everyone is as much a disadvantage as it is an advantage. (Yes, they all have default configurations of course, but you are generally expected to customize.) When people occasionally ask me about it, I tend to advise people to pick their tiled window manager less on the putative features and more by what programming language that it is implemented in that they are comfortable in. Now, if you're fluent in one of the languages in question, that's awesome, but it does raise the bar for a lot of people.

You don't have to be an "expert" in Haskell to use XMonad, but it sure doesn't hurt if you want to do some heavier-duty customizations, like use modules that aren't even nominally a part of XMonad.