Yup, been trying almost the same game engines as you, and more. Have to say C3 has something special. It just clicks with people once they try it. Is the simplicity of it all. The user interface makes sense, is all layered out in a way you don't have to think twice where objects and options are. Compared to unity interface or other engines. The UI learning curve is very short as mimics the old paint interface.
As features comes, most of what I need is there. I been using it for years now, and had my share points of complaints and disappointments regarding some of the objects and missing features, but being a web based engine has it's limitations, and with time i realized majority of my complaints where a fan Boy perspective and were not realistically pointing towards what i need as tools and features for what im creating.
The only thing that I'm having a struggle with is the learning curve over multiplayer game systems, however in it's defense it does say in fine print "requires some server side coding knowledge". Which compared to other engines is a breeze. But I'm learning it, so many tutorials from the community and the engine creators online. Wish there were more in depth tutorials covering step by step setting up IAP's and other stuff. And not have to read a e-book about it or a convoluted manual that at the end of it you realize it still didn't completely give you a answer to all of your questions. Luckily that missing information is found online from the awesome community creators that share their dev process, and we learn from one another. The community is somehow like Quora or stackoverflow for C3, which makes it shine.
Other than that, one missing feature i wish it had, is the browser loading interface to be actively customizable and not by having 2-3 loading screens but to basically hijack the loading bar in beginning and customize the graphics of it and all. Which is a bit hard to do as requires some work around and "dissecting" how the "start screen" happens before your "custom loader layout kicks in".
And a 3rd wish there was a C# or C++ compiler to convert the js code automatically to C# or C++ that would help massively the future of this engine, as would open the chance to be easily published and ported to Sony Devices such as PlayStation and XboX. There are 3rd parties that manually do it for you, but in the process they loose fine tuning you are doing on ur "game mechanics" and you lose a lot of time and money in the process of going back and forth with them to pin point exactly "the gravity feel" or other stuff you coded in C3.
Short to be said, some of this problems i have are probably most of the heavy users problems, that are looking to use C3 for creating amazing products to be published literally everywhere.
Maybe there are solutions to what i said. But after some 5+ years i sort of gotten tired of searching for answers and just focus on what i can achieve with the engine in it's current state. And by simplifying my expectations it just got more natural to use and less frustrating.
Amazing product, really the go to game engine when something needs to be done fast.