I recently had a support for a PrestaShop 184.108.40.206 where the cart rule was not taken into account in the order page, no matter what, the carrier price was still the same (instead of a free shipping).
I found out it came from the CartRule class of PrestaShop, more precisely from the country restriction.
Few weeks ago, PrestaShop announced the availability of PrestaShop 1.7 Beta 1 during the PrestaShop Day Paris. Finally developers can have a look at the new draft of PrestaShop, implementing Symfony 2 and Twig, but in the end, what does it change for developers ? For now on, not too much, only a few pages has been rewritten with Symfony and Twig so you most likely not going to be in trouble (yet).