For the last few days, i have been testing MATE 1.8 RC on a clean VM of Slackware 14.1 and so far, the results were great. No major problems found during quick test and it's well integrated with Slackware 14.1. I would like to give big credit for Patrick Volkerding for giving big support of MSB in Slackware 14.1, so that some of the packages needed in MSB could be integrated in the main tree.
I'm still using MATE 1.6 on my laptop and i will try to upgrade to MATE 1.8 as soon as they are released. I never tested on upgrade process, but i believe the risk is so minimum. The MATE developer didn't make an aggressive or massive update to the packages, just incremental updates, which is something i like. I hate BIG CHANGES like what happened with the latest systemd release: systemd 209. It causes lots of problem for downstream (Linux Distributions) as they have to take precautions about the massive changes and make sure that it will not break their product.
I'm so lucky that Slackware isn't using systemd (at least until 14.1) and hopefully, we will never have to battle against systemd in the future.