We'll set aside performance because that could be fixed with a renderer rewrite.

The biggest issue when I worked with it was weaving a spiderweb of bindings that eventually trap you. At some point, you wind up spending most of your time fighting weird bugs that show up in places very far removed from the bindings that actually caused the bug.

Unfortunately I never got around to trying it! I remember it being a contender around the time of Backbone, but then Ember and Angular were the main options thereafter.