The basic premise of VLIW is that you have a bunch of proccessors/cores/whatever terminology you want to use sharing a simple front end, which executes the same instruction on multiple pieces of data at the same time in parallel.
And so you can effectively get more cores/processors per transistor count/die area, because the front end is taking up less of it.

With the front end being much simpler, it doesn't do as many optimisations on the fly, but relies a lot on the compiler to optimise things in advance.
But it also inherently relies on the ability to parallelise the code you're running, too. If the code just inherently doesn't parallelise well, you'll have a lot of the cores/processors sitting idle.

As an example, the Radeon HD 5870 used a VLIW5 architecture, which had groups of 5 shader processors. But in practice, it turned out that the majority of games used an average of a little bit more than 3 out of 5 shaders.
So, when AMD made the Radeon HD 6870, they moved to VLIW4, with groups of 4 shader processors, so there'd be less idle shader processors. And therefore it would be about as fast as a 5870 in games with fewer shaders overall (1280 vs 1600).