AbleCommerce Checkout Custom Developement

[page8worker]

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,