Is not so bad really - I think I understand your objections, but - VST2 relies on C++ too, and, polymorphism and class inheritance are actually quite an elegant way to manage an extensible plug-in API. The main objection I would guess would be compatibility issues, which generally holds better with pure C than C++ (but mainly because the C library hadn't changed... much - until.. of course the recent gcc5 etc compatibility issues).... directly shipping C++ interfaces...
My plug-ins have always been C++ (even the old LV2 variants, because, as soon as you get into e.g. the GUI code, it makes complete sense to use an object oriented architecture and I didn't want to re-invent the C++ 'wheel' so to speak).
So far (not wanting to tempt fate) they have enjoyed good compatibility with different linux distros, host applications etc. If you are trying to build (especially binaries) to run on anyone else's system other than your own, it is a particularly special challenge on linux, and C++ is quite low down the list of issues.
I've actually just built a linux VST3 version of one of my plug-ins, and so far it seems to work just as well (without needing to use cmake, or requiring any vstgui / GTK dependencies - which is nice)