I used Nucleus for a while as my blog system. I admit, it was a nifty little piece of software. However, it did not have the inbuilt functionality I wanted: trackbacks, and several smaller features. Instead, these core functions had to be added through plugins. The Nucleus plugin system is quite a clever thing, but it isn't nearly clever enough to be used to add core functionality.
Another reason why I switched was because the templating system wasn't too easy to use, at all. In fact, it was downright confusing: the interface was jumbled and all over the place, and there was a distinct lack of inline documentation.
A further reason why I switched, was because it really isn't very good for a professional web developer to be using someone else's product to power his own website; he should really be creating his own product.