The only downside is it takes a while to get your head around its object model and how to structure your apps in terms of computed properties and observers. Once that clicks though your in a nirvana of productiveness, and having toyed with the react ecosystem I've got to say that Embers learning curve is a lot nicer.
There is Glimmer 2 comming out soonish, fast boot taking off and the Ember engines RFC implemented in the next version. Things are looking good!