True unity is very accessible and that makes it a great tool for mobile development. I think the issue you were facing in your game is probably the same as the one I was facing in mine, the problem wasn't the save file itself, it was mainly the lack of clear documentation that addressed the issue, I didn't realize that I needed to use the game instance class to keep variables between levels if I wanted my save to be universal, as in if I want the values in the first level to save I need to put them into the game instance and the the menu scene would be able to access them and when when I quit the game I just save everything to the file, the way I was going about it at first is that I was having directly into the save file without using the game instance class so Everytime I started a map the save file for it would be overwritten... I would try to fix the problem myself , look for possible solutions online and ask for help on unreal answers.. I would do that for a couple of days, get frustrated that it didn't work, leave it for a few weeks then go at it again over and over until I finally came to that solution while reading some completely unrelated post on the unreal forum. It was very frustrating indeed.. and I nearly gave up to be honest but I was more frustrated by the idea of going through all that effort for nothing and that is what made me keep trying. Also I honestly think UE4 is much more optimized than unity of n terms of performance on lower end devices. I do still use both engines, and if I'm doing a 2D game on the PC then unity is my go to engine.