Like any engine, Unity has its unique advantages and drawbacks. A small dev team can cover a lot of ground using Unity, and some programming tasks are actually quite pleasant with it. Some people have asserted in this thread that Unity's performance is deficient, but it's easy to write games that run smoothly on a large gamut of devices, whereas UnrealEngine's IDE and even almost empty scenes made with it run at 5fps on my maxed-out 2012 iMac. It might scale better, but Unity beats it handsdown in most low and medium complexity workloads.

These are reasons why Unity is well liked among indies, and not necessarily only among non-technical people either. But if you have many millions to burn, other engines give you more room for customization, have more high-class sales teams, and have a workflow that is optimized to be familiar to artists with a AAA background.

I've been developing a game using UE4 on a 2012 iMac for the past year and I get about 60fps@1080p in the editor on the highest settings. I can comfortably run DS+2 Clients to test multiplayer when I scale it down to Low. The only really slow thing is DebugDraw calls, which I haven't found to be a blocking issue, though it it certainly annoying.

I was amazed to discover that Firewatch (Unity) was pretty playable on my 2013 MacBook Air. I'm wondering though if that's a property of Unity itself, or more of a cultural thing. Could it be that those who use UnrealEngine just focus more on high-end games?