The last integration I did was a one way only (pulling orders from site to filemaker). It was Joomla with some ecommerce packs put on. I recall it was fairly straightforward to read the MySQL tables and translate it into FM. Probably 8-10 hours of work total.
I recall checking out a wordpress solution as well and turning it down due to the way the plugins were storing data. The thread earlier today about Magento was along the same lines.
All in all, it will probably depend a lot on the plugins you choose, not the platform itself. Just make sure the platform itself is easy to connect to (e.g. MySQL), and then evaluate a few different plugins by installing them and using something like Navicat to look at the tables and fields to see how comfortable you are.