You only need to look at the UK to see why coalitions are a bad idea in westminster systems. Bad for the minority coalition party anyways.
But is that because of the Westminster system or because of gross incompetence on the part of the junior partner in the current British government? Though it's certainly true that the Westminster system wasn't designed with coalitions in mind.
I think much of it was because a fair swag of the junior party's supporters weren't expecting a coalition with the Tories.
People didn't get that the Liberals aren't "Diet Labour".