I've been using Unity since day one and I'm currently on the fence as to which system to currently use between C3 and Unity in my next project. I used MMF2 fusion for my last project which was great, but suffers heavily in performance against the new C3 runtime.
Unity can do everything, but it's big and clunky these days and I seem to spend more time compiling, upgrading the editor or trying to find stuff(data, assets, settings..) within the complex UI rather than doing any actual game development. When things break its a case of deleting library folders and waiting 30 minutes each time to reload everything..This really ruins the flow. I was running Unity 2019 yesterday and it crashed on me about 5 times during the day.
C3 doesn't do any 3D (Big shame :() and has some small weaknesses against Unity 3D (no Spine support, fonts etc) but it seems super fast to develop and it runs super fast too, export sizes are something like 10-20 times smaller than Unity too.
ALso My Unity test Project is around 1 Gig (once I've added all the plugins etc) whereas my C3 test project with practically the same results is a couple of meg. It never crashed on me.
I only wish C3 did just a tiny bit of 3d. I'm still not sure if the C3 runtime fully supports skeletal animation though (Spriter/Spine)