i disagree with the assumption that a good tool will support all or even 90+% of use cases. you lose a lot by piling in feature after feature.
I much prefer a few different tools that do a few different things really well. I don't use active record if I have to deal with legacy schemas. I'll use data mapper or roll my own. If I get to design my own schemas then I love the benefits AR gives me.
As others say in this thread: ActiveRecord makes easy/simple things very easy. Unfortunately, it makes harder things damn near impossible. To aggravate matters, people that only use it for what it was designed for, tend to blame you, instead of acknowledging that it may very well be that their favorite tool doesn't support a particular kind of use.