I use quite a few add-ons and my e10 was disabled because of them. I force enabled e10 using Mozilla's guide https://wiki.mozilla.org/Electrolysis#Force_Enable and didn't notice anything breaking (yet).
It's usually less so things breaking and rather just being slow. For example, if you use NoScript, it often takes really long to load up a page, because that's when NoScript becomes active, and because NoScript isn't yet properly e10s-compatible, it just forces Firefox to do things in a single-process way. But that actually makes things slower than when Firefox is running without e10s.
about:performance is a rather good indicator, if an add-on is causing slow-downs.
