i believe when they made c3 one of the big things was they they completely rebuilt the engine, so while it works the same from a user standpoint i believe under the hood are changes; that being so, i would assume whatever they use to allow the plug ins to work in the first place would have to be recoded, which, i get it, why not just put that in the first time, however, constructs a fairly large ware with a lot of little things, so putting it together in parts makes sense, as other functions may have taken priority over cross compatability of plugins.
most of the plugins are currently or have already been converted, and a lot of the authors i would assume still use construct. though, it would be wise to contact authors, as, they may have update versions or knowledge of possible bugs or maybe even more plug in files. never hurts to ask.