It's easy, by comparison, to get to the 80% point with a game -- especially if you have a sample to start from that's somewhat close to what you want to create. What's hard is making every little bit work correctly; creating a finished product that doesn't have glitches, that works on all devices/in all browsers, and that has a good "feel" throughout. That last bit of polish, that last "20%", probably takes well over 80% of the time; I'd estimate it at about 95% of the time. And if you start with an "80%" engine that hasn't gone through that polish phase at least once, then you're doing all that work yourself.
This isn't precisely what you asked: You asked for success stories. A casual browsing of the site doesn't find any published game lists, though. The creators of Phaser.io do have a list of games THEY'VE published with it:
http://www.photonstorm.com/games
Look at the "HTML5" options; they used to make games in Flash.
I looked at Phaser a while back, and decided against using it. None of those games actually look much more complicated than a proof-of-concept, 80%-at-most demo.
On the other hand, Cocos2d-X has been used in dozens of hit games, and they are up to v3.0 of Cocos2d-js, which looks awesome, has an amazing free toolchain you can use, works in a browser, AND can be embedded in a custom wrapper on mobile (something like PhoneGap/Cordova, only tuned and optimized for game development -- it includes SpiderMonkey directly, for instance, so you don't have to worry about what browser it's running in).
The choice wasn't that hard.