* the uncertainty range immeadiately leaps up on the first day of prediction to completely implausible levels (as well as having an implausible lower limit too);
* the model has an implausible level of certainty that the whole thing will be over by the end of the first week in May;
* it's all a bit Gaussian looking1.
That's taken from https://covid19.healthdata.org/united-kingdom, if you want to look for your self.
I finally dragged myself out of my lethargy to read their paper and discovered that they don't go out of their way to tell you what their methods are. But if you read it, it's fairly plain: The cumulative death rate for each location is assumed to follow a parametrized Gaussian error function. So, that's their "modelling". But that's worthless, because epidemics don't follow a Gaussian, especially if they've got a lock-down in the middle of the data, whereupon fitting a Gaussian goes form being a bad idea to a cretinous one.
I'm guessing (though I haven't looked) that this explains their uncertainty bounds too: all they've done is taken the mean and fuzzed it, so the uncertainty is proportional to the value. Which is also worthless.
This also explains why the model goes to zero when it does: since we happen to look like we've got to the "top" of the Gaussian, it's simply predicting a mirror-image of itself as a decline. Also worthless.
But then we get people like Nate Silver Twitting "There are some good critiques of the IHME model in here IMO" and... it's all to wishy-washy. Yes there is in that one good point: According to a critique by researchers at the London School of Hygiene & Tropical Medicine and Imperial College London, published this week in Annals of Internal Medicine, the IHME projections are based “on a statistical model with no epidemiologic basis.” (my bold). And yet despite all this no-one can actually be bothered to read their paper and say what's wrong with it.
1. [2021/12] I may have been wrong to think this a flaw; see for example James "the natural infection profile of an uncontrolled epidemic".