Hi, I'm one of dev of Solitaire: Decked Out. :) ( Sorry, I'm not a native speaker, so my writing must be bit wierd ;) )
We developed this game mainly with Unity (a game engine). I personally don't recommend to use it for "casual game", cause it's too HEAVY (than cocos2dx, or native like swift).
Nobody wants a solitaire that drains battery, takes 100MB when installed, occasional slow down, and etc etc. And these bad things happen in Unity if you don't take care about engine's detail ;(
Unlike Unity, Cocos2dx and other lower level engines have much smaller resource usage. I mean CPU, GPU, memory, battery, storage, plus network!
BUT cocos2dx or other lower level engines have stiff learning curve, and "really dangerous" for novice cause default behaviour of exception is crash. ( It kinda trade-off, cause these engines access most resources in UNSAFE manner but fast )
(* I believe after that learning curve, we can build an app as fast as high level game engine that take care about every details other than actual game logic in SAFE way. )
So why did we decide to use this notorious Unity? (I'm actually familiar with cocos2dx, more than 3 years of experience)
It's because we are just 5 people team, and 2 are client programer (include me), and one is a junior who just started client programming this year. We are small team and have only limited budget, so if we wanna success, we need to move fast. In that sence, choosing cocos2dx or lower level engine is too risky, cause it took at least 3 MONTH to learn. And even if we handled that learning curve somehow, the quality of product can be bad! ( like occasional crash ) Cause we need to handle ALL thing, like from threading to resource allocatiom.
So my recommendation for making casual mobile game is,
If you are familiar with lower level engine, stick with that. If you are not, and if you have enough time and money ( which both can't be exist in this world except china ;) ), than learn lower level engine. If you are not, use high level engine like Unity.
Thanks.
PS. If you are looking for high-level, fast engine, than consider Defold which is the engine made by King.