AbleCommerce Shopping Cart Software

I'm currently evaluating AbleCommerce for one of my development projects. The project would require me to make some major modifications to the checkout process. So far it seems that I should have no problems making my modifications, but I was curious if anyone out there could tell me if there are any specific areas in AbleCommerce that a developer would not be able to customize (anywhere in the system, not just the checkout pages)?

Thanks,