The main critics is that it does not support type classes, but maybe that is actually not a bad thing. The discussion is similar to the never ending Go discussion about generics and people do amazing stuff with Go (now I guess critics would say they would do even more amazing stuff with generics).

Not that I say type classes (or generics) are useless, but keeping a language simple (in the sense of keeping the feature list small) has benefits on its own. I did not to complicated projects in Elm, but I watch the language closely and really like what I have seen so far, so I hope Evan keeps up the good work, as I think he has pretty good taste for language design.

It's not that it is not complete, but not entirely as powerful as Haskell which inspires a lot of how Elm works and looks like. The creator/maintainer of elm doesn't believe they're required.

FWIW, when this was new it got a lot of pushback in a related Reddit thread. Might be worth digging in if you're seriously thinking about it.

This was definitely a pain point for me when I experimented with Elm.