I’d refute that claim for two reasons. The first is simple: Shyamalan’s last film, The Visit, was excellent: funny, clever genre fare.
The second reason is that Split is not particularly good. I think it’s reasonably well-made – Shyamalan’s use of space, lighting and dolly zooms are notable, and McAvoy’s lead performance as the multiple-personality “Kevin” is certainly committed – but it fails as the kind of schlocky genre film it wants so desperately to be.
My primary objection to Split is that it’s, well, boring. The premise is a good ‘un, but Shyamalan’s screenplay spends too much time outlining over-exposition and clumsy foreshadowing through an unnecessary psychologist character (Betty Buckley, dreadful). It’s the kind of film that sounds silly and fun when synopsised but is a (near two hour!) drag in practice.
But what earns Split such a low rating is the way it represents mental illness. I don’t have an issue with its realism, but simply that its conclusion glamorises mental illness in a way that I, personally, found deeply discomforting. You – like Richard Brody – might have an alternative read, but it just rubbed me the wrong way.