I am not sure if anybody has used UE4 for a significant amount of time, as it is “young”. We have used it for two commercial projects which both turned out to be quite tedious due to us having to develop many components by ourselves. While we could achieve anything we wanted (of course, it’s straight-forward C++ and you can mess with anything), we would invest far more development time than scheduled based on our Unity experience. (And, mind me, we hired a couple of really talented C++ guys on top.) This year, UE4 was one option for as many as 6 commercial projects (most of them VR-projects outside of the gaming industry) and in the end none of the project teams chose to go with UE4. 5 teams would go with Unity, one is extending an in-house solution.
And yes, UE4 is “younger” and therefore Epic devs as well as third parties didn’t have a whole lot of time to extend the kit beyond the basics (and then some), while the Unity team had more time to learn, adopt, extend - plus of course the tons of valuable assets you can buy for literally nothing. Not all of these assets are gold, but we regularly save hundreds of development days by utilizing/extending one of the great libraries. I understand that hobby coders will not have the funds to do so (but then, they also do not have the time to invest many man-years in developing the same functionality by themselves), and I understand that huge studios with hundreds of devs can invest in developing all functionality by themselves; yet we, as a mid-sized studio, can only invest the majority of our budget in the actual content / mechanics / (game) logic.
Nevertheless, so many statements in this Unreal (!) forums are simply wrong. Stating that UE4 looks better simply means that the specific author didn’t bother to look at Unity for years; if you e.g. use Substance Designer (which apparently >85% of the studios do), your substances will look the same in both kits. Stating that Unity is inferior because it doesn’t have a material editor built in natively is simply whining. You can choose between several AAA material editor solutions from the asset store for a couple of dollars. Stating that UE4 is a game engine while Unity 5 is not … well, I am not even going into that … good joke. Stating that Unity is for beginners while UE4 is for professionals is wrong too - the only complex thing about game development is happening deep inside your code logic, and that’s the same because you need to build the same. Claiming that C++ is so much more complex than C# makes me yawn. If you coded less than 20 years in C++ and less than 10 years in C# you should be in professional game development anyway. And if you did, both is straight-forward and really not that hard. Oh, and another favorite statement is that in Unity you need to code a ton of tools before you can actually start … wrong too. You might pick up a dozen useful helpers from the Asset Store, you hook up your repository system, and you are good to go. Anything you might adjust on the way (e.g. your own layouts, etc.) can be transferred to your other projects easily. Anybody stating you need to develop tools might have (a) not understood the concepts, or (b) might not have spent an hour in the tools section of the Asset Store to pick out a couple of goodies. Final favourite wrong statement: UE4 blueprints make UE4 so much superior to Unity. Just look at the Asset Store again: there’s several (!) node base visual scripting tools made by industry veterans that will make you cry happy tears. Just pick what fits your development/design style best, and even adjust/extend the code if needed.
Ultimately, it only comes down to how quickly you can achieve what your bosses / producers ordered, in a stable way. Looking at UE4 - basically at ANY component of game dev - you are always looking at a mountain of manual work you need to do … UI, AI (behavior trees, etc.), whatever. The reason is not that Epic devs wouldn’t have a good concepts or “forgot” to add it. I am sure many of these things are written down on tiny to-do-stickers on some Product Managers backlog board. It’s simply that UE4 in it’s current form is not on the market since a long time, and that’s why it will take time for Epic to include it. After they did: knock yourself off with any devkit you want to use (in the end 99% of your projects quality will depend on your talent, skill and creativity anyway), and until then pick your battle wisely. Your wrong assumption of “UE4 looks better” might cost you thousands of hours more in development.