Really? We've implemented completely custom designs on top of Foundation for several sites now and haven't had any problems. Then again, we work to get the layout the way we like it with Foundation before we start designing, maybe that's why.
For us, things like buttons required a ton of overriding with Bootstrap when we customized the button design, but Foundation didn't. We found this largely to be the case because Foundation makes less assumptions about how buttons are styled to begin with. For example, buttons are pretty basic with square corners. If you want rounded corners, you just add the "round" class. If you want buttons with a different radius, you just create your own new class with a specific border radius and add that. Different color? Create a new class with a background color and add that.
What in Foundation did you experience pain trying to override?
For us, things like buttons required a ton of overriding with Bootstrap when we customized the button design, but Foundation didn't. We found this largely to be the case because Foundation makes less assumptions about how buttons are styled to begin with. For example, buttons are pretty basic with square corners. If you want rounded corners, you just add the "round" class. If you want buttons with a different radius, you just create your own new class with a specific border radius and add that. Different color? Create a new class with a background color and add that.
What in Foundation did you experience pain trying to override?